Moving lektors default branches from 'master' to 'main'

As we move across the Tor Project from 'master' to 'main' we need to move the different websites. This ticket will gather the process needed to move the lektor-based websites, as they all follow a similar path.

For this to happen, we need to

• Move the actual branches
• Adapt gitlab to the new branches

Lektor-based websites status regarding master/main transition:

• gettor-web
• support
• tb-manual
• tpo (tpo#90 (closed))
• community
• dev
• styleguide
• donate
• newsletter

Lektor-based websites regarding builds/publishing

• gettor-web
• support
• tb-manual
• tpo
• community
• dev
• styleguide
• donate
• newsletter