[PodSecurity] Warn when adding PSA labels to exempt namespaces

[tallclair]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Return a warning when adding or updating a non-privileged Pod Security label on an exempted namespace.

• Adding or updating a version label on a privileged namespace doesn't warn, but doing so on a non-privileged NS does

The exempt audit annotation is no longer included on update namespace requests on exempt namespaces. It's usage was inconsistent (didn't apply to create), and is less relevant to namespace operations (we're not bypassing the policy).

Which issue(s) this PR fixes:

Fixes #109549

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Return a warning when applying a pod-security.kubernetes.io label to a PodSecurity-exempted namespace.
Stop including the pod-security.kubernetes.io/exempt=namespace audit annotation on namespace requests.

/sig auth
/milestone v1.25
/assign @liggitt

[k8s-ci-robot]

Please note that we're already in Test Freeze for the release-1.24 branch. This means every merged PR has to be cherry-picked into the release branch to be part of the upcoming v1.24.0 release.

[k8s-ci-robot]

@tallclair: This issue is currently awaiting triage.

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[tallclair]

/retest

[tallclair]

/retest

[liggitt]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, tallclair

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Needs approval from an approver in each of these files:

• staging/src/k8s.io/pod-security-admission/OWNERS [liggitt,tallclair]

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[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[tallclair]

Trivial test fix, adding back lgtm.

[k8s-triage-robot]

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/retest