ci: use GitHub merge queue (#40130)

Requiring branches to be up to date reruns the full CI matrix on every
waiting PR after each merge, which saturates our 20 concurrent jobs. A
merge queue still tests against the latest `main`, but once per batch.

This runs the PR workflows on `merge_group` and reports `giteabot/lgtm`
on queue commits, since GiteaBot only sets it on PR heads. Mergers queue
PRs with "Merge when ready" instead of `reviewed/wait-merge`, so
giteabot no longer updates branches. Nothing changes until the
`merge-queue` ruleset is enabled and "Require branches to be up to date"
is turned off. Backport to `release/v28` before the ruleset covers it.

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Co-authored-by: silverwind <me@silverwind.io>

.github/workflows/pull-db-tests.yml

@@ -2,6 +2,7 @@ name: db-tests
 
 on:
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}