Upgrade from bullseye (bullseye-backports went EOL today)

[glebm]

We use devkitpro (for Switch) and devkitARM (for 3DS) containers on our CI.

This stopped working today as we can no longer install packages from bullseye-backports, such as cmake:

E: The repository 'http://deb.debian.org/debian bullseye-backports Release' no longer has a Release file.

This is because bullseye-backports has been discontinued earlier today: https://backports.debian.org/news/bullseye_backports_discontinued/

CI build failures:

• 3DS: https://github.com/diasurgical/DevilutionX/actions/runs/16482040465/job/46598595673?pr=8079
• Switch: https://github.com/diasurgical/DevilutionX/actions/runs/16482040479/job/46598595736?pr=8079

/cc @AJenbo @StephenCWills