this helps to avoid build issues when virtual environments are different between forks of the same repository
e.g., ubuntu-latest means 20.04 in upstream while it still points to 18.04 in a fork
Mainly to allow using 'if constexpr'.
Older CI compiler setups like GCC 5 for Linux were removed as a consequence.
Windows 32 bit was also removed because there are no plans to do any more 32 bit releases of GZDoom.
