Updates to GA version of PodDisruptionBudget

[psschwei]

Fixes #11488

Proposed Changes

As release v1.2 will require a minimum Kuberenetes version of 1.21, it's time to update the PodDisruptionBudget API.

/hold
for v1.21 release (though we should probably merge before the release)

Release Note

PodDisruptionBudget updated to v1 API

[codecov]

Codecov Report

Merging #12548 (3985059) into main (f953230) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main   #12548   +/-   ##
=======================================
  Coverage   87.48%   87.48%           
=======================================
  Files         195      195           
  Lines        9720     9720           
=======================================
  Hits         8504     8504           
  Misses        931      931           
  Partials      285      285           

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[dprotaso]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, psschwei

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[psschwei]

/hold cancel

[psschwei]

/test pull-knative-serving-upgrade-tests

[psschwei]

/retest