ubuntu-22.04: switch to GCC 12
Ubuntu 22.04 has GCC 12 available, so let's use that instead of GCC 11. When we write extensive build documentation, we should mention that users also need to set CXX=g++-12 and CC=gcc-12 during meson setup.
Ubuntu 22.04 has GCC 12 available, so let's use that instead of GCC 11. When we write extensive build documentation, we should mention that users also need to set CXX=g++-12 and CC=gcc-12 during meson setup.