Remove the CI jobs that tested with Dune 2.8

I don't think we have to support Dune 2.8 anymore.

added 3 commits

• 8643ecff - Update the Debian 10 CI jobs to Debian 11
• 74d4ab03 - CI: Don't patch dune-common anymore to work around a buggy ADOL-C
• 86991ccf - Raise CMake requirements to 3.16

Compare with previous version

merged

mentioned in commit c2ac2169