Add reviewers and maintainers for GitLab project declarative-policy

Why is this change being made?

In https://about.gitlab.com/handbook/engineering/projects/#declarative-policy we don't list any reviewers and maintainers.

This MR adds all backend engineers from ~"group::authentication and authorization" (taken https://about.gitlab.com/handbook/product/categories/#authentication-and-authorization-group) and myself as explicit maintainers.

Note that in the past, several maintainers have reviewed and merged MRs in that very project. However, it'd be great to have a list of reviewers/maintainers (also to be used by Reviewer Roulette) on the project page to highlight the ownership.

Refs gitlab-org/ruby/gems/declarative-policy#27 (closed).

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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