Add Roy Zwambag as gitlab backend maintainer

Manager Justification

It's hard to specify hard requirements for becoming a maintainer, which is why the documentation consists of flexible guidelines. Reviewers are encouraged to think of their eligibility for maintainership in the terms of "I could be ready at any time to be a maintainer as long as my work as an author and reviewer is consistent with other maintainers".

• The MRs reviewed by the candidate consistently make it through maintainer review without significant additionally required changes.
• The MRs authored by the candidate consistently make it through reviewer and maintainer review without significant required changes.

@rzwambag joined GitLab and started contributing to GitLab projects about 15 months ago. He started the trainee maintainer training 5 months ago. In the trainee maintainer issue, Roy has demonstrated the ability to consistently provide good code reviews and the maintainers also have been giving feedback which helped Roy become a better reviewer. I think it's the time for Roy to step up as a gitlab backend maintainer. I am creating this MR to make the change.

Before Merging (Manager Tasks)

• Mention the maintainers from the given specialty and ask them to provide feedback to the manager directly.
• Leave this merge request open for 1 week, to give the maintainers time to provide feedback.
• Ensure we have at least 2 approvals from existing maintainers.

Once This MR is Merged

1. Create an access request for maintainer access to gitlab-org/<project>.
2. Join the #backend_maintainers Slack channel
3. Ask the maintainers in your group to invite you to any maintainer-specific meeting if one exists.
4. Let a maintainer add you to @gitlab-org/maintainers/rails-backend
5. Announce it everywhere
6. Keep reviewing, start merging 🤘 😎 🤘