Update the default GNAT version from 9 to 11, as GNAT >= 11 is required to compile the 22.* AdaCore libraries. To allow this, we need to pick a patch from ghdl's master fixing a compilation problem with GNAT 11.
Moved to ./contributing-to-this-manual.chapter.md. Link:
https://nixos.org/manual/nixos/unstable/#chap-contributing