[compiler-rt] Move CODE_OWNERS.txt to Maintainers.txt (llvm#118086)

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

No format change, I just want each project to have a `Maintainers.*`
file so it is easy to find.

Author: DavidSpickett

compiler-rt/CODE_OWNERS.TXT renamed to compiler-rt/Maintainers.txt

@@ -430,7 +430,7 @@ Others only have a lead maintainer listed here.
 
 [Clang-tools-extra maintainers](https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/Maintainers.txt)
 
-[Compiler-rt maintainers](https://github.com/llvm/llvm-project/blob/main/compiler-rt/CODE_OWNERS.TXT)
+[Compiler-rt maintainers](https://github.com/llvm/llvm-project/blob/main/compiler-rt/Maintainers.txt)
 
 [Flang maintainers](https://github.com/llvm/llvm-project/blob/main/flang/Maintainers.txt)