[9.0] (backport #17745) Moved dependabot reviewers to codeowners

[mergify]

Release notes

[rn:skip]

What does this PR do?

Removed reviewers section in dependabot.yml and added CODEOWNERS.yml with related definition for observablt-ci && observablt-ci-contractors

Why is it important/What is the impact to the user?

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

• [x ] 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 (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

Related issues

• Relates: ci: move from dependabot reviewers to CODEOWNERS #17744

Logs

---

This is an automatic backport of pull request #17745 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e9a3b7e has failed:

On branch mergify/bp/9.0/pr-17745
Your branch is up to date with 'origin/9.0'.

You are currently cherry-picking commit e9a3b7ed.
  (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)

Changes to be committed:
	new file:   .github/CODEOWNERS

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/dependabot.yml

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