Add validation for "cencel" of "taskrun" and "pipelinerun"

[vincent-pli]

Changes

Need validation when cancel taskrun and pipelinerun.

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Release Notes

When cancel a Pipelinerun or Taskrun, user should update spec with exactly "PipelineRunCancelled" and "TaskRunCancelled" or the webhook will reject the request with ErrInvalidValue and error message.

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1beta1/pipelinerun_validation.go	95.5%	96.0%	0.5
pkg/apis/pipeline/v1beta1/taskrun_validation.go	97.2%	97.4%	0.2

[ghost]

/approve

[vdemeester]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg, vdemeester

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[bobcatfish]

thanks for fixing htis @vincent-pli ! request: could you possibly fill in the release notes section in the PR description? i could imagine this impacting users if they were putting the wrong value in this field and suddenly validation will catch it

[vincent-pli]

@bobcatfish
Done, thanks.

[vincent-pli]

/test pull-tekton-pipeline-integration-tests