Add Terri Chu as backend maintainer

Trainee maintainer issue:

Overview

• I have worked at GitLab since March 2020 and have had 146 MRs merged across gitlab-org, gitlab-com, and gitlab-elasticsearch-indexer.
• I am also a reviewer for the gitlab-elasticsearch-indexer
• I've participated as a trainee backend maintainer for approximately 9 months and have reviewed 156 MRs as part of the program.
• I have participated in the trainee backend program and meet with my mentor every 6 weeks

Examples of reviews

Note: I'm working on this list, pulling from the trainee issue

• I caught an issue in unchanged code in this MR: #9656 (comment 660499705)
• I asked some good questions which resulted in another issue/MR being opened which improved performance in multiple areas: #9656 (comment 615243893)
• this was an interesting review for me because it was in an area that i have little experience: #9656 (comment 580035429)

Things to improve

@gitlab-org/maintainers/rails-backend please chime in below with your thoughts, and approve this MR if you agree.

Developer checklist

• Before this MR is merged
  • Mention @gitlab-org/maintainers/rails-backend, if not done (this issue template should do this automatically)
  • Assign this issue to your manager
• After this MR is merged
  • Request a maintainer from the #backend_maintainers Slack channel to add you as an Owner to gitlab-org/maintainers/rails-backend
  • Consider adding 'backend maintainer' to your Slack notification keywords

Manager checklist

• Before this MR is merged
  • The MR has been open for 5 working days
  • More than half of the existing maintainers approve the MR
  • There are no blocking concerns raised (if there are, please follow https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-project-maintainer)
• After this MR is merged
  • Announce the good news in the relevant channels listed in https://about.gitlab.com/handbook/engineering/#keeping-yourself-informed