Change the GDK default branch name from master to main

Overview

As part of GitLab's effort to rename Git's default initial branch epic, we want to change the default branch for the GDK from master to main.

Steps

1. Announce on #gdk that we're about to start
2. Jump on GDK Zoom call
3. Make the master branch read only
4. Disable merge trains if necessary
5. Merge !1885 (merged) / !1890 (merged)
6. Set main as the default branch
7. Perform new GDK installs and update existing installs to test
8. Update WIP MRs to have their target branch set to main
9. Re-enable merge trains if necessary
10. Announce on #gdk that we've cutover 🙂