[llvm] [github] Change "code owners" note to refer to "maintainers" (PR #118087)

[David Spickett via llvm-commits]

DavidSpickett wrote:

To be landed once https://github.com/llvm/llvm-project/pull/118082, https://github.com/llvm/llvm-project/pull/118084 and https://github.com/llvm/llvm-project/pull/118086 remove the remaining CODE_OWNERS.txt files.

https://github.com/llvm/llvm-project/pull/118087