How to sync 'main' into 'experimental'

Every time a new commit reaches main, the CI automatically creates/updates a PR called "chore: sync main to experimental" and leaves it open for a last manual check and manual merging.

To make the CI run the other CI steps (build, etc.) on that PR, we have to close and re-open the PR being a verified dev account. That is because the CI commit in the middle is not signed (we're looking into improving this)

The PR has to be COMMIT-MERGED not SQUASH-MERGED: for that you have to temporarily enable the commit-merges in the settings of the repo, and then disable it again just in case.