[clang-tools-extra] Rename CODE_OWNERS -> Maintainers (PR #114544)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

@PiotrZSL The ping was in case you want to be added as a maintainer to clang-tidy as well. I’m mentioning it just to make sure.

https://github.com/llvm/llvm-project/pull/114544