deprecate label triage/build-watcher and associated job

[dhiller]

We maintain our GitHub label definitions in the labels.yaml - the definitions in GitHub are updated periodically after any change to that file.

The label triage/build-watcher has not been used for more that three years 1 - it is currently superceded by the sig/ci label, which is tracking qurantine and infra stuff. Thus we should add a deprecation notice.

Suggestion: prefix the description 2 of the label with DEPRECATED:, which should make it clear that this label should not be used any more.

The commenter periodic job definition 3 can then be removed.

/kind tracker /priority backlog