PRs targeting release branches can receive duplicate "Thanks for creating this PR!" comments.

The reason for this is drift between the develop and release branch versions of pr_checklist.yml. The develop version runs on `pull_request_target` while the release branches run on `pull_request`, so the task runs twice, once for each pull request event type.

Impact: Minor annoyance during GitHub PRs. The bot will post the same comment twice. The comment is not a blocker for merging a PR, so it's not a CI blocker.

Fix: Backport commit 6fb83cc46f to the release branches, or manually change `on: pull_request` to `on: pull_request_target`.