Add script to check for missing backports

There should be a script (and optionally interactive GitHub Action flow for easy usage via the web UI) which:

• Takes a release version as input (e.g. 1.6-next)
• Fetches all JIRA issues with that fix version
• Verifies that each of the issues is referenced by at least one commit on the corresponding branch in git (e.g. 1.6)

The purpose is to identify any "forgotten" backports of commits. Note this would not identify the situation where a subset of multiple commits attributed to one issue got backported but another subset were not, but that seems rather unlikely and having that basic check described above is a substantial improvement over the current situation.