ci: move from dependabot reviewers to CODEOWNERS

[oakrizan]

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/

Requested in https://github.com/elastic/observability-robots/issues/2767