DMD --> GDC swith for Dlang
Please remove DMD compiler (it is too slow) - and add GDC (GCC backend) for Dlang

There is some clumsy way to install all that we need for D testing (LDC, GDC and DUB) - please check the comment here https://github.com/dlang-community/setup-dlang/issues/35#issuecomment-753548770

And could you consider an opportunity to upgrade CI workflow from Ubuntu 20.04 to 22.04?
Becase fresh GDC available only starting from that version of Ubuntu.