Support all locations of `CODEOWNERS` file

[Alexander-Shukaev]

As per GH docs:

To use a CODEOWNERS file, create a new file called CODEOWNERS in the .github/, root, or docs/ directory of the repository, in the branch where you'd like to add the code owners. If CODEOWNERS files exist in more than one of those locations, GitHub will search for them in that order and use the first one it finds.

https://github.com/mergeability/mergeable/blob/62481abcaa15166f21a75b375310912080922564/lib/validators/options_processor/owners.js#L98