Post-release tweaks (cross-version testing cleanup, release checklist tweaks)

With the v9.19.16 release tag merged, the "cross-version-config-tests" GitLab CI job will no longer fail due to the two relevant system tests being absent from the development branch. This makes the pytest filtering expression added to work around that issue unnecessary, so remove it.

Update the release checklist with links to recently implemented tools for automating certain tasks.