- Added gcc-12 and clang-13 through 15 jobs.
- Added C++23 testing for gcc and clang on Linux.
- Updated clang version for UBSAN job.
- Updated Ubuntu version for clang jobs to avoid having to use external APT
repository.
- Updated python package installation for compatibility with Ubuntu 22.04.
In Ubuntu 20.04 there appeared an updated version of the
software-properties-common package in focal-updates, which ships a newer
apt-add-repository version that doesn't support -P/-S/-U command line arguments.
Since we cannot rely on package version checks to determine apt-add-repository
capabilities, we have to parse its --help output instead.
Also, made source list processing more protected against spaces.
