Exclude namespace-banned users from being counted as billable members

What does this MR do and why?

• Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/203
• Excludes members that are banned in a namespace from being counted as billable members

Screenshots or screen recordings

Screen_Recording_2023-03-12_at_9.03.16_pm

How to set up and validate locally

1. Add any user as a member to a parent group, eg. Gitlab Org on http://localhost:3000/groups/gitlab-org/-/group_members
2. Calculate the number of billed users for that namespace. The user you just added should be included in that response

bundle exec rails c

> billed_users = Namespace.find_by(name: 'Gitlab Org').billed_user_ids[:user_ids].count
> billed_users[:user_ids].count
> reload!

2. Ban the user from that namespace.

3. The user would have now moved to the Banned section on the members page, http://localhost:3000/groups/gitlab-org/-/group_members?tab=banned
4. Calculate the billed members of that namespace again. The user you just banned should be excluded from that result.

Query Plans with gitlab-org namespace

Method	Buffers (current)	Buffers (with this query)
billed_group_users	32357	32363
billed_project_users	19083	24460
billed_shared_group_users	32484	33176
billed_invited_group_to_project_users	47875	49338

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.