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

[Piotr Zegar via cfe-commits]

PiotrZSL 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.

The designation of "maintainer" does not impact the established process. This role primarily introduces additional responsibilities without offering any demonstrable advantages. It is likely to become dead in the near future, mirroring the fate of the code owner role. At the end, being or not being a maintainer has no impact on me.

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