extensions lib: Drop the EnsureKubeAPIServerService func from the controlplane Ensurer

[ialidzhikov]

How to categorize this PR?

/area scalability
/kind cleanup

What this PR does / why we need it:
See the motivation in #9755

Which issue(s) this PR fixes:
Fixes #9755

Special notes for your reviewer:
N/A

Release note:

The `github.com/gardener/gardener/extensions/pkg/webhook/controlplane/genericmutator.Ensurer#EnsureKubeAPIServerService` func is removed. This func was used before the introduction of `ManagedIstio`/`APIServerSNI` (when the kube-apiserver Service was of type LoadBalancer) to set cloud provider specific annotations to the Service. However, after `ManagedIstio`/`APIServerSNI` are unconditionally enabled (the kube-apiserver Service is of type ClusterIP) this func is no longer used. Nowadays, istio-ingressgateway Service annotations can be provided via the Seed spec.

With the removal of the `github.com/gardener/gardener/extensions/pkg/webhook/controlplane/genericmutator.Ensurer#EnsureKubeAPIServerService` func, the provider extensions using the `genericmutator.Ensurer` no longer need to mutate Services and should no longer mutate Services to prevent no-op webhook invocations.

[plkokanov]

/lgtm

Should we also add text to the release note to instruct extension maintainers that https://github.com/gardener/gardener-extension-provider-aws/blob/aaf1ad53dfb45a14c63662f36611bb239f32bc2d/pkg/webhook/controlplaneexposure/add.go#L41 should be removed when revendoring this change for extensions not in the gardener org: e.g. https://github.com/metal-stack/gardener-extension-provider-metal/blob/6dfa3254b70484c68dcd1705ad159fa9e978bbb2/pkg/webhook/controlplaneexposure/add.go#L37

[gardener-prow]

LGTM label has been added.

Git tree hash: f1f14e546dec62ad4ea4ae988e194bfe35b169f5

[ialidzhikov]

Should we also add text to the release note to instruct extension maintainers that https://github.com/gardener/gardener-extension-provider-aws/blob/aaf1ad53dfb45a14c63662f36611bb239f32bc2d/pkg/webhook/controlplaneexposure/add.go#L41 should be removed when revendoring this change for extensions not in the gardener org: e.g. https://github.com/metal-stack/gardener-extension-provider-metal/blob/6dfa3254b70484c68dcd1705ad159fa9e978bbb2/pkg/webhook/controlplaneexposure/add.go#L37

Makes sense, thanks! I now added a new release note to the PR. For the provider extensions in the gardener org I raised the corresponding PRs - see gardener/gardener-extension-provider-aws#952 and all linked PRs.

[rfranzke]

Thanks for cleaning up :)
/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rfranzke]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment