Remove v1beta1 Pipeline API support

[lcarva]

Changes

This commit removes all of the support to the long-deprecated v1beta1 version of the Pipeline API.

Submitter Checklist

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

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

v1beta1 Pipeline API is no longer supported.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/artifacts/signable.go	73.4%	70.4%	-3.0
pkg/chains/formats/slsa/extract/extract.go	66.7%	76.4%	9.7
pkg/chains/formats/slsa/internal/material/material.go	91.0%	90.1%	-0.9
pkg/chains/formats/slsa/internal/resolved_dependencies/resolved_dependencies.go	84.6%	83.3%	-1.2
pkg/chains/formats/slsa/v1/intotoite6.go	26.7%	61.5%	34.9
pkg/chains/objects/objects.go	22.0%	42.7%	20.8
pkg/chains/storage/gcs/gcs.go	64.2%	70.1%	5.9

[lcarva]

The linter failures all seem to point to a single line which appears to be bogus?

[PuneetPunamiya]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: PuneetPunamiya

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