A few days ago GNAT-LLVM was updated to support LLVM v21, nice! This is the commit: Make GNAT LLVM compatible with LLVM 21 · AdaCore/gnat-llvm@778d5a8 · GitHub
The built-in LLVM builder script still points/uses to v19, but you should be able to use v21 now! Nice!
Best regards,
Fer