Move PodReadinessGates feature gate out of validation

[rajathagasthya]

What type of PR is this?

/kind api-change
/kind bug

What this PR does / why we need it:
Moves feature gate checking of pod ReadinessGates out of validation into strategy utility methods, and avoids dropping data on update if the existing pod spec already used ReadinessGates. Adds unit test for the strategy utility method and updates validation test.

Which issue(s) this PR fixes:
xref #72651

Does this PR introduce a user-facing change?:

The `spec.readinessGates` field is now dropped during creation of `Pod` objects unless the `PodReadinessGates` feature gate is enabled.

/sig api-machinery

[k8s-ci-robot]

Hi @rajathagasthya. Thanks for your PR.

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[rajathagasthya]

/assign @liggitt

[liggitt]

one nit, lgtm otherwise, thanks!

[liggitt]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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