Removed MacOS 12 and added MacOS 15 job as the former image is removed
from GHA.
Added newer gcc and clang jobs on Linux. Updated Linux images to reduce
reliance on external package repositories.
- 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.