Display group members only when setting allowlist for a group

What does this MR do and why?

Resolves: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/64

Depends on: !93336 (merged)

This MR updates the allowlist input in Git abuse rate limit group settings to only display users that are members of the group (vs displaying all users).

Screenshots or screen recordings

Before	After
Screen_Recording_2022-08-11_at_10.18.31_AM	Screen_Recording_2022-08-11_at_10.19.22_AM

How to set up and validate locally

Set up

1. Ensure you are running GDK with an GitLab Ultimate license. The following command should log true if this is set up correctly

   echo "License.feature_available?(:unique_project_download_limit)" | rails c

2. Turn on the feature flag:

   echo "Feature.enable(:limit_unique_project_downloads_per_namespace_user)" | rails c

Validate

1. Create a top-level group (or use an existing one)
2. Using the group owner as user, navigate to the groups Settings > Reporting page
3. Validate that the allowlist input only displays users that are members of the group

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.