…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

…server_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

* Allow instantiating v1.26 Kubernetes clients

* Update `README.md` and `docs/usage/supported_k8s_versions.md` for the K8s 1.26

* Maintain Kubernetes feature gates

$ ./hack/compare-k8s-feature-gates.sh 1.25 1.26
Feature gates added in 1.26 compared to 1.25:
APISelfSubjectReview
AggregatedDiscoveryEndpoint
ConsistentHTTPGetHandlers
CrossNamespaceVolumeDataSource
DynamicResourceAllocation
EventedPLEG
LegacyServiceAccountTokenTracking
MinimizeIPTablesRestore
PDBUnhealthyPodEvictionPolicy
PodSchedulingReadiness
StatefulSetStartOrdinal
TopologyManagerPolicyAlphaOptions
TopologyManagerPolicyBetaOptions
TopologyManagerPolicyOptions
ValidatingAdmissionPolicy
WindowsHostNetwork

Feature gates removed in 1.26 compared to 1.25:
CSIMigrationOpenStack
CSRDuration
DefaultPodTopologySpread
DynamicKubeletConfig
IndexedJob
NonPreemptingPriority
PodAffinityNamespaceSelector
PodOverhead
PreferNominatedNode
ServiceLBNodePortControl
ServiceLoadBalancerClass
SuspendJob

Feature gates locked to default in 1.26 compared to 1.25:
CPUManager
CSIMigrationvSphere
DelegateFSGroupToCSIDriver
DevicePlugins
DryRun
EndpointSliceTerminatingCondition
JobTrackingWithFinalizers
KubeletCredentialProviders
MixedProtocolLBService
ServerSideApply
ServiceIPStaticSubrange
ServiceInternalTrafficPolicy
WindowsHostProcessContainers

* Maintain `kube-apiserver` admission plugins

$ ./hack/compare-k8s-admission-plugins.sh 1.25 1.26
Admission plugins added in 1.26 compared to 1.25:
ValidatingAdmissionPolicy

Admission plugins removed in 1.26 compared to 1.25:

* Maintain `ServiceAccount` names for the controllers part of `kube-controller-manager`

$ ./hack/compare-k8s-controllers.sh 1.25 1.26

kube-controller-manager controllers added in 1.26 compared to 1.25:
resource-claim-controller

kube-controller-manager controllers removed in 1.26 compared to 1.25:

* Use 1.26 for local shoot and local e2e test

* Deprecate the `podEvictionTimeout` field in favor of newly introduced kube-apiserver fields

The kube-controller-manager flag `--pod-eviction-timeout` is deprecated in favor of the kube-apiserver flags `--default-not-ready-toleration-seconds` and `--default-unreachable-toleration-seconds`. The `--pod-eviction-timeout` flag does not have effect when the taint besed eviction is enabled. The taint  based eviction is beta (enabled by default) since Kubernetes 1.13 and GA since Kubernetes 1.18. For more details, see kubernetes/kubernetes#74651.

This commit allows configuring the kube-apiserver flags `--default-not-ready-toleration-seconds` and `--default-unreachable-toleration-seconds`. The `podEvictionTimeout` field is deprecated in favor of the newly introduced fields. gardener-apiserver no longer defaults the `podEvictionTimeout` field. gardener-apiserver also returns a warning when the `podEvictionTimeout` field is set.

* Adapt to the renaming of `etcd_db_total_size_in_bytes` metric to `apiserver_storage_db_total_size_in_bytes`

The metric `etcd_db_total_size_in_bytes` is renamed to `apiserver_storage_db_total_size_in_bytes`. Ref kubernetes/kubernetes#113310.

* Fix the Pod spec in `simple-load-deployment.yaml.tpl`

Test runs of the integration test that uses this template prints the following warning about the issue in the template:
```
{"level":"info","ts":"2022-12-28T19:36:29.043+0200","logger":"KubeAPIWarningLogger","msg":"unknown field \"spec.template.spec.containers[0].nodeName\""}
```

* Update `docs/usage/shoot_credentials_rotation.md`

After the removal of support for Kubernetes < 1.20 Shoot clusters (ref #6987), the kubeconfig Secret no longer has the `token` field. Basic auth cannot be enabled for K8s 1.19+ clusters, hence the kubeconfig Secret cannot contain the `username`/`password` fields anymore.

* Default `enableStaticTokenKubeconfig` to false for Shoots with K8s version >= 1.26

This commit also adapts most of the testmachinery integration tests to use the `shoots/adminkubeconfig` subresource instead of the static kubeconfig.
The Shoot creation intergration is still using the static kubeconfig and it is downloading it to `$TM_KUBECONFIG_PATH/shoot.config`. This commit sets `enableStaticTokenKubeconfig=true` until we figure out which tests/components are using this downloaded kubeconfig.

* Add constraint for K8s version < 1.26

The constraint `ConstraintK8sLess126` is currently not used by gardener/gardener but it is introduced for usage from the extensions.

* Address review comments

* Update `new-kubernetes-version.md` guide with details about the `hyperkube` image

* Update `supported_k8s_versions.md` for K8s 1.26

* Update kube-scheduler component's unit tests for K8s 1.26

* Revert the K8s versions used for e2e tests

For the reasoning, see #7275 (comment)