Add Bala 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.

@bala.kumar is part of the release team for more than a year and has authored 72 MRs in the GitLab project since then.

As part of his backend maintainer training, he has reviewed 26 MRs. The most recent ones were all merged as-is without additional comments from maintainers. Bala does thoughtful reviews. In his suggestions, he explains why a change can improve the code and includes examples, which makes it easy to understand his intentions and makes the review more efficient. Some examples that stand out are:

• gitlab-org/gitlab!92607 (merged) is a good example of a well handled community contribution. The comments included concrete code suggestions and explained why a suggestion was made and how it can improve the code.
• gitlab-org/gitlab!93993 (merged) Bala showed that he considered the bigger picture and found that the change also requires a frontend change.
• gitlab-org/gitlab!93161 (merged) Bala showed deep Ruby on Rails knowledge by suggesting including an argument scope.
• gitlab-org/gitlab!92164 (merged) - A JiHu contribution

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 group
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 🤘 😎 🤘