[bolt] Move CODE_OWNERS.txt to Maintainers.txt

To align with: https://llvm.org/docs/DeveloperPolicy.html#maintainers

I have not changed the format of the file, my only goal
here is that the project have a `bolt/Maintainers.*` so it
is easy to find.

Author: DavidSpickett

bolt/CODE_OWNERS.TXT renamed to bolt/Maintainers.txt

@@ -424,7 +424,7 @@ [email protected] (email), [gkistanova](https://github.com/gkistanova) (GitHu
 Some subprojects maintain their own list of per-component maintainers.
 Others only have a lead maintainer listed here.
 
-[Bolt maintainers](https://github.com/llvm/llvm-project/blob/main/bolt/CODE_OWNERS.TXT)
+[Bolt maintainers](https://github.com/llvm/llvm-project/blob/main/bolt/Maintainers.txt)
 
 [Clang maintainers](https://github.com/llvm/llvm-project/blob/main/clang/Maintainers.rst)