TEP-0114 Serve Custom Task Beta

[XinruZhang]

Changes

This PR reverts #5736 because we've aligned on TEP-0121 about retries.

/kind cleanup

Submitter Checklist

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• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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Release Notes

Custom Task Beta is available.

[XinruZhang]

/assign @abayer @lbernick

[lbernick]

I think the webhook needs to be updated as well

[pritidesai]

Ready for review, please take a look, thanks!

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel, lbernick

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[abayer]

/lgtm