[TF-M] Rename TF-M branches master -> main

Hello,

Some time ago we planned to rename our master branch to main in all 6 repositories. There are multiple ways of doing that and I want to discuss with the community the best way suitable for all. I can see the following options:

1. rename master->main and use the main from one day. 2. Create the main for contribution with main->master sync. Make master R/O and delete it in 3 months. 3. Keep contributing to the master but syncing master->main. 4. Other way?

I suggest option 2 but looking for better alternatives if any.

Thanks, Anton