Reduce number of jobs and increse minimal required compiler versions
This MR reduces the number of default jobs to four:
- ubuntu:20.04 gcc-9 c++17
- ubuntu:20.04 clang-10, c++17
- debian:11 gcc-10, c++20
- debian:11 clang-11, c++20
with ubuntu 20.04 has cmake 3.16, debian-11 has cmake 3.18 (debian-12 has cmake 3.24)
So, we have an old setup with a c++17 compilers and two relative recent compilers that already support c++-20 concepts. Also there is a gcc and a clang compiler with possibly different behavior.
Note, this MR increases the minimal compiler versions from gcc-7 to gcc-9. This was discussed in a dev meeting. clang versions arer raised to clang-10. Since we didn't decided to increase also the minimal c++ standard, there are still two jobs with c++-17
Edited by Simon Praetorius