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

[Aiden Grossman via llvm-commits]

boomanaiden154 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.

Sounds reasonable enough to me. In the comment I also mentioned we should upgrade if there are relevant `clang-format` backports because I think that is something we would probably want.

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