Update CI images, project deps and tests optimizations
Ci, tests and dependency maintenance MR.
Since we've upgraded mentors.d.n to bookworm, we no longer need to support bullseye. As such:
- The CI images have been updated to drop bullseye and include trixie
- The dependencies are now aligned with bookworm versions, mainly bumping django requirements from 2.x to 3.x.
Additionally, I've modified the way of running tests to be able to run concurrently. On my box, this brings down running the entire test suite from 7:30s to 2:10s.