Add support for LLVM 21

[nathanchance]

• Update LLVM main anchor and version to 22 (pull request)
• Wait for LLVM 21 binaries on kernel.org (done)
• Wait for TuxMake to provide korg-clang-21 images (pull request, done)
• Wait for TuxSuite to allow korg-clang-21 to be specified
• Update generator/yml to provide llvm_20 anchor and copy all llvm_latest builds to it
• Update generator/yml to shift llvm_latest to 21 and copy all llvm_tot builds to it