Change default branch name

With respect to Update default branch in active toolbox projects (gitlab-com/support/support-team-meta#5228 - closed) I think this project is probably the most delicate one. It's something we constantly reference, which many users know and use themselves. Unfortunately the default branch name is also hardcoded into various usage instructions, and I know from experience that some customers even built SOS automation which we could break.

Considering that this is not an officially supported tool, I don't think we have any obligations but I'm sure we all want to try and avoid disruption.

I propose that we:

1. Create a new branch main based of the current master
2. Change the default branch to main
3. Apply the current protections for the master branch to the new main branch
4. Change all occurrences of master to main in the usage instructions (mainly this Readme, but also make sure to check for any macros/snippets that might include a full URL)
5. Wait x amount of time (12 months should be plenty)
6. Then remove the master branch

Optionally we could still update the master branch with the latest main from time to time, but I think having it be a stable version is okay as well. This really is just a fallback, I expect most usage to change to main very quickly.

• Steps 1 to 3 are trivially done within a minute.
• Step 4 is a little more work, but not time critical as master will continue to work. I'll start working on it once steps 1-3 are done and main is available.
• For steps 5 and 6 I'd open a follow-up issue with corresponding due date.