CI: Use Alpine instead of Ubuntu
Ubuntu 20.10 is now totally unsupported, and the Alpine tests should ensure better version coverage, and allow us to use specific versions of Dulwich instead.
This instance will be upgraded to Heptapod 17.5.3 on 2024-12-12 between 10:00 and 11:00 UTC+1
Ubuntu 20.10 is now totally unsupported, and the Alpine tests should ensure better version coverage, and allow us to use specific versions of Dulwich instead.