Deprecate legacy retry:when failure reasons

What does this MR do and why?

Follow-up to !239415 (merged), which made the legacy stuck_or_timeout_failure and job_execution_timeout reasons behave as aliases in retry:when matching. This MR adds the user-facing deprecation pieces that were intentionally left out of that change:

• CI config warning: Gitlab::Ci::Config::Entry::Retry now emits a non-blocking warning when either legacy reason appears under retry:when, pointing users at the specific reasons that replaced them. Warnings surface wherever existing CI config warnings do — CI lint, the lint API, and the pipeline editor.
• Deprecation notice: a new entry under data/deprecations/, announced in %19.1 and slated for removal in %20.0, plus the regenerated doc/update/deprecations.md.
• Docs: the retry:when reference in doc/ci/yaml/_index.md now notes the deprecation and lists the replacement reasons for each legacy value.

The warning is non-blocking, so existing pipelines keep running unchanged — the alias behavior from !239415 (merged) still does the actual retrying.

References

• Resolves #602133 (closed)
• Follow-up to !239415 (merged)