TEP-0114: Remove enable-custom-task and alpha feature flag

[JeromeJu]

Changes

This commits remove the enable-custom-task and alpha feature flag for the
custom task feature gates.

/kind feature
Fixes: #5155

Submitter Checklist

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

• [n/a] Has Docs included if any changes are user facing
• [n/a] 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
• [n/a] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [n/a] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[tekton-robot]

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[JeromeJu]

/ok-to-test

[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/apis/config/feature_flags.go	81.8%	82.7%	0.9
pkg/reconciler/pipelinerun/resources/pipelinerunresolution.go	94.3%	94.2%	-0.0

[jerop]

/hold

@JeromeJu thank for for this PR - this will be one of the last steps in implementing TEP-0114 - let's follow the ordering in #4313 🙏🏾

[JeromeJu]

/retest

[tekton-robot]

@JeromeJu: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-tekton-pipeline-build-tests	8e406c0	link	true	/test pull-tekton-pipeline-build-tests
pull-tekton-pipeline-alpha-integration-tests	8e406c0	link	true	/test pull-tekton-pipeline-alpha-integration-tests
pull-tekton-pipeline-integration-tests	8e406c0	link	true	/test pull-tekton-pipeline-integration-tests

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[JeromeJu]

/hold

@JeromeJu thank for for this PR - this will be one of the last steps in implementing TEP-0114 - let's follow the ordering in #4313 🙏🏾

Thanks @jerop for the pointer. Will be following #4313

[tekton-robot]

@JeromeJu: PR needs rebase.

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[vdemeester]

I think prior to removing, we should change the default value (to true) and deprecate the option so that we remove it in a couple of releases. This will give time for our users to upgrade at their pace.