[PROJECTMGMT] Rename 'master' to 'main' branch

[rzwitserloot]

We'd like to follow programming trends unless there's a good reason to deviate from them. Seems like the default name for the common deployment branch in git setups is to be 'main'; ours is currently 'master'.

I'd like to change this. But how does this impact others? Does this require manual actions by any and all forks and PRs in progress? Is 5 steps to change github default branch from Steven Mortimer a good guidebook? Are there pressing reasons not to change names?

This is, as far as I can tell, fairly low priority, so we'll wait a while for feedback.