[llvm] [Github] Bump clang-format to 21.1.0 (PR #155660)

[via llvm-commits]

owenca wrote:

So are we updating the clang-format version twice for each release cycle? See https://github.com/llvm/llvm-project/pull/141819#issuecomment-2921073176.

https://github.com/llvm/llvm-project/pull/155660