[llvm] [bolt] Move CODE_OWNERS.txt to Maintainers.txt (PR #118082)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Note that this file is not used by GitHub to assign reviewers, as explained here - https://github.com/llvm/llvm-project/blob/5540eac4e5161d124ac547e4a9d3451408bb71e9/.github/CODEOWNERS#L10

So moving it doesn't break anything.

(I will remove that note once all projects have moved to `Maintainers.*`)

https://github.com/llvm/llvm-project/pull/118082