remove SendCloudEventsForRuns from PipelineProperties once upstream pipeline drops the field

Summary

The send-cloudevents-for-runs feature flag was deprecated in tektoncd/pipeline v1.12.0
(tektoncd/pipeline#9774).
CloudEvents for CustomRuns are now enabled by default when a sink is configured in the
config-events ConfigMap.

The SendCloudEventsForRuns field in PipelineProperties was marked deprecated in #3430,
which also updated the default from false to true to align with upstream behaviour.

Prerequisite

This removal must only happen after send-cloudevents-for-runs is fully removed from
tektoncd/pipeline.

Track upstream removal progress here:

• Upstream deprecation: tektoncd/pipeline#9774
• Watch for the upstream issue/PR that removes the field entirely from tektoncd/pipeline
  before acting on this issue.

What needs to be done (after upstream field is removed)

• Confirm send-cloudevents-for-runs is fully removed from the upstream
  tektoncd/pipeline release being consumed by the operator
• Remove SendCloudEventsForRuns field from PipelineProperties struct in
  pkg/apis/operator/v1alpha1/tektonpipeline_types.go
• Remove the corresponding SetDefaults logic in
  pkg/apis/operator/v1alpha1/tektonpipeline_defaults.go
• Update / remove affected unit tests
• Update documentation (docs/TektonPipeline.md) to remove references to
  send-cloudevents-for-runs
• Add a release note entry

Notes

• This field only ever affected CustomRun objects; it has no effect on TaskRun or
  PipelineRun.
• Users who set send-cloudevents-for-runs: false to suppress duplicate events from a
  custom task controller that already sends its own CloudEvents should be made aware via
  release notes when this removal lands.
• See similar precedent: removal of the deprecated verification-mode field tracked in
  #1497.

References

• Upstream deprecation PR: tektoncd/pipeline#9774
• Operator deprecation / default fix: #3430

First verify that send-cloudevents-for-runs and its upstream field are fully removed from the tektoncd/pipeline release consumed by the operator. Then inspect pkg/apis/operator/v1alpha1/tektonpipeline_types.go and tektonpipeline_defaults.go, update affected unit tests, remove the reference from docs/TektonPipeline.md, and add a release note. Done means the deprecated field and defaulting logic are removed without affected tests or documentation references remaining.