Update dependabot and CODEOWNERS file

[MaciejKaras]

Summary:

Updates dependabot.yml and CODEOWNERS to fix issue:

Dependabot tried to add @slaskawi, @lsierant, @mircea-cosbuc, @nammn and @Julien-Ben as reviewers to this PR, but received the following error from GitHub:

POST https://api.github.com/repos/mongodb/mongodb-kubernetes-operator/pulls/1657/requested_reviewers: 422 - Reviews may only be requested from collaborators. One or more of the users or teams you specified is not a collaborator of the mongodb/mongodb-kubernetes-operator repository. // See: https://docs.github.com/rest/pulls/review-requests#request-reviewers-for-a-pull-request

All Submissions:

• Have you opened an Issue before filing this PR?
• Have you signed our CLA?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Put closes #XXXX in your comment to auto-close the issue that your PR fixes (if such).