Add Matt D'Angelo as a backend maintainer

Manager Justification

• I have been working on the GitLab project for over 9 months as part of groupproduct planning
• I have reviewed and approved over 70 MRs, including those from outside groupproduct planning, and including a community contribution
• Here is my trainee issue, which includes non-trivial MRs reviewed by me
• I have been heavily involved in the rollout of several major features in the groupproduct planning group, including the work items powered project issues list and group epics list

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 and ask them to provide feedback to the manager directly. Emphasize that any negative feedback should be communicated privately to the manager/mentor, not in the merge request, as outlined in our maintainership feedback documentation.
• Leave this merge request open for 1 week, to give the maintainers time to provide feedback.
• Check this box to confirm that you approve the access provision.
• 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://handbook.gitlab.com/handbook/engineering/workflow/code-review/#request-maintainership-feedback) please share it with me privately.
* If you are in agreement, and can vouch for this proposal, please approve.

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. Remove yourself from maintainer_mentorship.yml and consider adding yourself as a maintainer mentor.
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 <project>/maintainers/backend with Owner access level.
5. Announce it everywhere
6. Familiarize yourself with documentation for Merging a merge request
7. Keep reviewing, start merging 🤘 😎 🤘