Consolidate validation for Task/Pipeline beta features

[lbernick]

Currently, the validation webhook includes code for validating that "enable-api-fields" is set to "beta" when a v1 object is created using beta features.

Validation of Task and Pipeline specs is currently applied in the reconciler as well, to referenced Tasks and Pipelines that are fetched remotely or from the cluster. This commit refactors validation of beta features in Tasks and Pipelines, so that it is all done in one place. There are no functional changes in this commit, but it will make it easier to fix issues related to versioned validation of referenced Tasks and Pipelines.

/kind cleanup
Part 1 of #6616

Submitter Checklist

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

• n/a 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
• 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). See some examples of good release notes.
• 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]

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/pipeline/v1/pipeline_validation.go	99.7%	99.7%	0.0
pkg/apis/pipeline/v1/task_validation.go	97.3%	97.3%	0.0

[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/apis/pipeline/v1/pipeline_validation.go	99.7%	99.7%	0.0
pkg/apis/pipeline/v1/task_validation.go	97.3%	97.3%	0.0

[JeromeJu]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dibyom, JeromeJu, Yongxuanzhang

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• pkg/apis/OWNERS [dibyom]

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[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/pipeline/v1/pipeline_validation.go	99.7%	99.7%	0.0
pkg/apis/pipeline/v1/task_validation.go	97.3%	97.3%	0.0

[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/apis/pipeline/v1/pipeline_validation.go	99.7%	99.7%	0.0
pkg/apis/pipeline/v1/task_validation.go	97.3%	97.3%	0.0