Make dstull customers app backend maintainer

Overview

Overall GitLab experience: 13 months and 430+ MRs merged.

Projects maintainer on:

• Versions application with 170+ MRs merged and 70+ MRs reviewed/approved
• License application with 30+ MRs merged

Customers app:

• authored MRs
• considerable contributions to overall health and maintainability of the application through
  • gitlab-org/customers-gitlab-com#670 (closed)
  • gitlab-org/customers-gitlab-com#1787 (closed) (ongoing)
  • gitlab-org/customers-gitlab-com#1791 (closed)
  • gitlab-org/customers-gitlab-com#1792 (closed) (almost finished)
  • gitlab-org/customers-gitlab-com#1852 (closed)
  • gitlab-org/customers-gitlab-com#1911 (closed)
• refactoring initiatives
  • gitlab-org/customers-gitlab-com#1796 (closed)
  • gitlab-org/customers-gitlab-com#1908 (closed)
  • gitlab-org/customers-gitlab-com#1959 (closed)

Developer checklist

• Before this MR is merged
  • Mention https://about.gitlab.com/handbook/engineering/projects/#customers-app backend maintainers, if not done
  • Assign this issue to your manager
• After this MR is merged
  • Request a maintainer from the #customers-gitlab-com Slack channel to add you as an Maintainer to https://gitlab.com/gitlab-org/customers-gitlab-com
    • not sure this is the right channel...

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

/cc @jameslopez @rdavila @asubramanian1 @tyleramos @shreyasagarwal