Upgrading to 1.8, we hit an issue where the PipelineRun can no longer be deleted due to

admission webhook "validation.webhook.pipeline.tekton.dev" denied the request: decoding request failed: cannot decode incoming old object: json: unknown field "serviceAccountNames"

serviceAccountNames was deprecated and then removed in the 1.8 release. Updates were made to the pipelines to remove this, but the old PipelineRun objects can not be deleted.

Relevant upstream issue around validation on delete:
https://github.com/tektoncd/pipeline/issues/5317