dependabot drops reviewers field

[pahatz]

As it is described here and in all recent PRs, dependabot will drop the field reviewers in dependabot.yaml and instead use github's CODEOWNERS.

• The reviewers sections in the dependabot.yaml need to be removed.
• A .github/CODEOWNERS file is added that contains the correct owners for the repo.
  https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-file-location

More info: https://github.blog/changelog/2025-04-29-dependabot-reviewers-configuration-option-being-replaced-by-code-owners/