Add release 2.9 images
I have added the releases 2.9 jobs, but reduced the docker images to 3:
- debian-10, gcc-8, c++17
- debian-11, gcc-10, c++20 (default)
- debian-11, clang-11, c++20
Edited by Simon Praetorius
I have added the releases 2.9 jobs, but reduced the docker images to 3: