Ensure `master` branch between Canonical and Security can diverge

As part of &109 (closed)

We need to be able to merge security merge requests once they're ready. Security commits will only live on Security until the Security Release is published

This implies master will diverge between Canonical and Security.

To do

• ✅ - Configure merge train to update Security master with Canonical master
• ⏳ - Investigate automatic toggling of merge train based on mirror status

auto-deploy branches between Canonical and Security are updated through merges