CI Failure: 404 Not Found when installing clang-format

[infrat]

The "Clang-Format 21 Check" workflow is currently failing during the installation step.
MOST LIKELY it's caused by runner operating on ubuntu-22.04 (Jammy), but the workflow is configured to fetch packages from the Debian 11 (Bullseye) LLVM repository (llvm-toolchain-bullseye-21).

[infrat]

Ok, it turns out this is a known upstream issue with the LLVM APT repository.
llvm/llvm-project#182462

We could wait for fix, or switch to stable version (clang-format-20) instead.