chore(rt-v1): retire promote-release-candidate.yaml (DEVEX-1656)

[pdodgen-revparts]

Context

RT v2 has landed successfully. Retiring the v1 promote orchestrator per docs/promote-release-candidate-retirement.md in encodium/actions.

Simplified from the retirement doc

The retirement doc originally called for three PRs — two per-caller cleanup (webstore + internal_api Start Release Train.yaml) plus this delete. Both callers are already gone — Start Release Train.yaml doesn't exist in webstore or internal_api. So this collapses to one PR: the delete.

Verified via per-workflow grep across all 22 RT participants plus encodium/.github and encodium/actions. Nothing invokes this file.

Rollback

Revert this PR. RT v1 was never destructive — the legacy rc branches, *-main.N tags in git history, and mathieudutour release_branches overrides in each participant continue to exist even without this orchestrator. This workflow was purely orchestration on top of state that lives elsewhere.

Companion work

• 22 draft PRs across every RT participant scrubbing rc gate references + Jellyfish notify jobs (in-flight — see task ci: update docker layer caching action #59 in the sweep report)
• encodium/terraform#3471 — remove rc branch + protection blocks for manage + internal_api (draft)
• Follow-up delete of jellyfish-deploy-notification.yaml after every caller PR lands
• gh api DELETE rc branches on the ~10 repos where they exist, after the participant sweep merges

[cursor]

PR Summary

Low Risk
Deletes unused CI orchestration with no runtime app impact; rollback is reverting the file if something still depended on it.

Overview
Removes the entire .github/workflows/promote-release-candidate.yaml reusable workflow (~200 lines), retiring the Release Train v1 “promote -main.N → pretty RC tag” orchestrator.

That workflow previously resolved the latest (or explicit) *-main.N tag, derived the pretty version, force-pushed the rc branch, retagged container images (including optional extra refs), optionally synced S3 CDN assets, and created a prerelease GitHub release. No replacement is added in this diff — promotion is handled elsewhere under RT v2.

Per the PR context, callers were already removed from participant repos; this change only deletes the shared workflow definition.

^{Reviewed by Cursor Bugbot for commit 60a4fe6. Bugbot is set up for automated code reviews on this repo. Configure here.}