Remove enable-api-fields validation for array index replacements

[lbernick]

Prior to this commit, the PipelineRun and TaskRun reconcilers checked whether "enable-api-fields" was set to "alpha" or "beta" before performing array param index replacement..
There were several problems with this logic:

• Beta CRDs don't require "enable-api-fields" to be "beta" to use beta features.
• This validation depends on the API version of the CRD that was created, so it belongs in the admission webhook rather than the reconciler.
• If "enable-api-fields" was set to "stable", param replacement was skipped. This means a user could create a valid array indexing expression in a beta CRD, and the reconciler would not correctly replace this param with its value.

This commit removes this logic.
/kind bug

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

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/reconciler/pipelinerun/resources/apply.go	98.1%	98.1%	-0.0
pkg/reconciler/taskrun/resources/apply.go	99.2%	99.2%	-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/reconciler/pipelinerun/resources/apply.go	98.1%	98.1%	-0.0
pkg/reconciler/taskrun/resources/apply.go	99.2%	99.2%	-0.0

[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/reconciler/pipelinerun/resources/apply.go	98.1%	98.1%	-0.0
pkg/reconciler/taskrun/resources/apply.go	99.2%	99.2%	-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/reconciler/pipelinerun/resources/apply.go	98.1%	98.1%	-0.0
pkg/reconciler/taskrun/resources/apply.go	99.2%	99.2%	-0.0

[Yongxuanzhang]

/lgtm

[tekton-robot]

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