Add Rajendra as 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.

Rajendra (@rkadam3):

Rajendra joined GitLab 2 years ago and was a community contributor before that for more than a year. He started working towards his maintainership under @splattael's mentorship.

1. Reviewed 238 MRs.
2. Uses Conventional Comment format, as per our guidelines, to convey intention.
3. Distinguishes between blocking and non-blocking suggestions for efficient review cycles.
4. Ensures our coding and testing guidelines are followed by linking them during reviews.
5. Checks if specs are added to ensure test coverage.
6. Shares to reuse existing code if it was missed earlier in the MR.
7. Asks questions if something is unclear.
8. Requests help from a domain expert or maintainer when he is unsure.
9. Provides review within the review-response SLO or let the author know about his unavailability.
10. Examples with no or minor suggestions by the maintainer: 1, 2, 3, 4
11. Helps triage a spec failure and work collaboratively.

Before Merging (Manager Tasks)

• Close any relevant trainee maintainer issues with a comment indicating that this merge request is being created, as (they are no longer required to become a maintainer).
• Mention the maintainers from the given specialty with the template below.
• 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.

Template call to action

SPECIALITY Maintainers, please review this proposal to make TRAINEE maintainer of PROJECT. 

* If you have blocking feedback adhering [to the documentation](https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-project-maintainer) please share it with me via Slack.
* If you are in agreement please approve or give a 👍 on the MR

After 1 week, if there is no blocking feedback and at least 2 approvals, I will merge this MR.

Once This MR is Merged

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