ci: serialize update-version-tags runs to close back-to-back-push race

PR #1953 added github.sha to the concurrency group key to prevent stale
"Re-run jobs" replays from cancelling the in-progress current-HEAD run.
Copilot review on the sagitta backport (#1955) caught the regression
that introduced: per-SHA groups mean back-to-back pushes A then B run in
parallel rather than serializing, and if run-A's force-PATCH lands
after run-B's, the tag rewinds to A.

Fix: per-branch group + cancel-in-progress: false.

- Concurrent runs serialize, so commit order is preserved on the tag.
- Stale "Re-run jobs" replays queue behind the current run, then hit
  the HEAD-equivalence guard in the job body and exit 0 — the guard
  (added in PR #1953) is the safeguard for that case, not the
  concurrency group.
- Tag-move work is fast (~5s); serial execution under back-to-back
  push bursts is acceptable.

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