Deprecate `master` in favor of `main`

Note: when we tackle this we'll need to modify some Github workflows. Ex

branches: [ master ]

Protected branches will also need to be updated

Since master is already protected, I think if we rename master to main, then main will remain protected but worth double checking when we tackle this.

FYI I just renamed master => main. @celestiaorg/celestia-core

Confirmed branch protection rules apply to main now.