[8.18] (backport #8709) Replace Dependabot reviewers with code owners

[mergify]

What does this PR do?

Removed reviewers section in dependabot.yml, since it duplicates the definition from CODEOWNERS.

Why is it important?

Reviewers dependabot.yml configuration is being retired option because the functionality overlaps with GitHub code owners.
See: https://github.blog/changelog/2025-04-29-dependabot-reviewers-configuration-option-being-replaced-by-code-owners/

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

Related issues

• Relates: ci: move from dependabot reviewers to CODEOWNERS #8708

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #8709 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4837940 has failed:

On branch mergify/bp/8.18/pr-8709
Your branch is up to date with 'origin/8.18'.

You are currently cherry-picking commit 48379406b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/dependabot.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally