Rename git master to main

Following tpo/core/team#2 (closed) as a template: let's (finally) do this.

We don't have this problem in the following projects:

• Tor Browser
• Fenix
• Android Components

We should rename the default branch of:

• Tor Browser Build
• RBM
• Torbutton
• Tor Launcher
• Tor Android Service
• Tor Browser Bundle Testsuite
• Tor Browser Spec

See also:

• tpo/tpa/gitlab#75 (closed)
• tpo/web/tpo#90 (closed)
• https://gitlab.com/anarcat/scripts/-/blob/master/git-rename-to-main