Branch 'master' will be renamed to 'main' on October 1, 2025

[mhucka]

To follow https://github.com/github/renaming as well as Google's own guidance, we plan on renaming the default branch (master) to main. This issue is announce the planned change and track miscellaneous tasks that need to be done around it. The date of the switch-over will be October 1, 2025.

Tasks:

• Check all files for mentions of master and update them to refer to main instead
• Check existing issues and PRs for possible impact of the change
• Check CI workflows & update as needed
• Rewrite GitHub branch protection rules as needed
• Contact past contributors to notify them & provide info about how to update their git clones

[mhucka]

This has been done.