Fix incorrect namespacing of `Resolvers::ValidateCodeownerFileResolver`
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=541916) </details> <!--IssueSummary end--> Currently the `Resolvers::ValidateCodeownerFileResolver` is incorrectly namespaced in the `EE` namespace. This namespace is reserved for CE extensions but this resolver is an EE only class not a CE extension. While we're moving this we should also fix the naming. It should either be ValidateCodeowner**s**FileResolver or ValidateCode**O**wnersFileResolver. Either way makes sense IMO. We're either validating the `CODEOWNERS` file or the file for the `Code Owners` feature. As we can't uppercase the whole word I'd personally lean towards `ValidateCodeOwnersFileResolver` The main thing here is the namespacing so wouldn't worry to much about the naming.
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