Add total group count to admin users table [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds the total group count for users in the admin users table as described in #276215 (closed). The following changes are made:

1. Adds total group count for users in the admin users table (!60998 (88ca88ad))
2. Removes the user_group_counts feature flag (!60998 (610c0626))
3. Add end-to-end feature spec for group counts (!60998 (0756d2af))

Note that this is only available in the Vue table &4961 (closed).

Backend implementation issue: #249589 (closed)

Setup & Testing

1. Enable the feature flags: echo "Feature.enable(:vue_admin_users)" | rails c
2. View the Groups table column at [GDK_HOST]/admin/users

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required) Updated GraphQL docs, no screenshots of table to update
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #276215 (closed)