[release-v0.53.x] Do not require enable-api-fields: alpha for entrypoint cancellation

[vdemeester]

Changes

This commit removes the need for enable-api-fields: alpha to enable the entrypoint cancellation. This feature should only behind the keep-pod-on-cancel: true feature-flag.

(cherry picked from commit 1b075fb)

This cherry-picks #7430 manually (as it conflicted).

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
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• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Entrypoint cancellation only requires `keep-pod-on-cancel: true` feature-flag.

[vdemeester]

/kind bug

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/pod.go	92.2%	92.7%	0.5
pkg/reconciler/taskrun/taskrun.go	85.6%	85.1%	-0.5

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/pod.go	92.2%	92.7%	0.5
pkg/reconciler/taskrun/taskrun.go	85.6%	85.1%	-0.5

[JeromeJu]

/lgtm

[tekton-robot]

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