Add Bojan as GitLab backend maintainer

Manager Justification

• 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.

@bmarjanovic joined GitLab 7 months ago, and he has been focusing on maintainership from the very beginning. @splattael was helping him during his journey as a trainee maintainer mentor.

• He has authored more than 75 merge requests.
• He has reviewed and approved more than 135 merge requests.
• He has an active trainee maintainer issue.

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 🤘 😎 🤘