Enable release/update pipeline in Mullvad Browser

Targets:

• 12.0a6 (03-17) build should have updater enabled
• 12.0a7 (03-24) build should include mars and update responses
  • once deployed 12.0a6 should be able to update to 12.0a7
• 12.0a8 (03-31) tentatively the last scheduled alpha before release
• 12.0.4 (04-??) first public stable release
• 12.5a4 (04-??) first public alpha release