gardener-apiserver keeps serving /openapi/v2 endpoint

[rfranzke]

How to categorize this PR?

/area usability
/kind regression

What this PR does / why we need it:
When gardener-apiserver does not serve the /openapi/v2 endpoint, users without permissions to list CustomResourceDefinitions might see the following error when using a kubectl version lower than 1.27 and trying to interact with the Gardener APIs:

error: error validating "github.com/gardener/gardener/example/provider-local/shoot-workerless.yaml": error validating data: failed to check CRD: failed to list CRDs: customresourcedefinitions.apiextensions.k8s.io is forbidden: User "system:serviceaccount:default:foo" cannot list resource "customresourcedefinitions" in API group "apiextensions.k8s.io" at the cluster scope; if you choose to ignore these errors, turn validation off with --validate=false

Introduced with #8464

Special notes for your reviewer:
/cc @acumino @dguendisch @timuthy @ary1992

Release note:

A bug has been fixed that prevented users without permissions to list `CustomResourceDefinition`s from interacting with the Gardener APIs when using a `kubectl` version lower than `1.27`.

[rfranzke]

/cherry-pick release-v1.80

[gardener-ci-robot]

@rfranzke: once the present PR merges, I will cherry-pick it on top of release-v1.80 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v1.80

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[acumino]

Thanks!
/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: 62f7fe9c56c74add86589574df9096e0b11a46f6

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: acumino

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[gardener-ci-robot]

@rfranzke: new pull request created: #8580

In response to this:

/cherry-pick release-v1.80

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.