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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> LGTM in regard to my details.
> 
> There are some emails that have their `@` escaped. Was that intended?

Yes, but it was a bad intention. :-D This is a .txt file, so we don't need to escape the `@` like we do in Clang (in Clang it's a .rst file because it's hooked in to our public documentation). I'll remove the escapes, thanks!

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