allow pipeline runs whose task/custom runs have been deleted still timeout

[gabemontero]

Expected Behavior

Back with PR 5134 Tekton started using the cancel function for cleaning up
underlying TaskRuns of PipelineRuns that are timing out, though separate
timeout functions were used (presumably in case the timeout behavior needed to be
tweaked from the cancel behavior later on). Then, in PR 5288, improvements to the baseline cancel function were made to still complete cancel processing of a
PipelineRun if the underlying TaskRuns were deleted. Presumably, the same would occur with timeouts, which use cancel under the covers.

Actual Behavior

PipelineRuns that timeout, but whose underlying TaskRuns have been deleted, get stuck with the success condition being "unknown" and the reason code `PipelineRunCouldntTimeOut

Steps to Reproduce the Problem

1. create a pipelinerun with a timeout, and then set it up so it does not complete in that timeout (add a long sleep for example)
2. delete its taskrun(s)/customrun(s) while it is sleeping
3. see what happens when the timeout starts

Additional Info

• Kubernetes version:

various versions

• Tekton Pipeline version:

0.53.x and forward, minimally

I'll be submitting a PR that strives to address this momentarily

@vdemeester @afrittoli @abayer FYI