Split array param indexing validation between reconciler and webhook

[lbernick]

There are two components to validation of array param indexing:

1. Validating that beta features are enabled.
2. Validating that array indexes are in bounds.

Prior to this commit, both of these forms of validation were done in the PipelineRun reconciler. However, the first form of validation differs between the v1beta1 and the v1 API version. In addition, array index bounds checking was skipped if beta features were disabled. This meant that if "enable-api-fields" was set to "stable", a user could create a beta Task/Pipeline using array parameter indexing, and we would not validate whether indexes were in bounds, potentially leading to a confusing error message or reconciler panic.

This commit moves the first form of validation to take place only in the webhook, leaving validation for bounds checking in the reconciler. Bounds checking is performed regardless of what feature flags are enabled.

/kind bug
Partially addresses #6616
Closes #6102
Related: #6607

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
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Release Notes

bug fix: always perform validation of array parameter index bounds checking

[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	99.4%	99.7%	0.3
pkg/apis/pipeline/v1/task_validation.go	96.8%	97.3%	0.5

[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	99.4%	99.7%	0.3
pkg/apis/pipeline/v1/task_validation.go	96.8%	97.3%	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/apis/pipeline/v1/pipeline_validation.go	99.4%	99.7%	0.3
pkg/apis/pipeline/v1/task_validation.go	96.8%	97.3%	0.5

[Yongxuanzhang]

/lgtm

[JeromeJu]

Thanks Lee

[tekton-robot]

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