Add check to prevent banning paid users

What does this MR do and why?

This MR adds a check to prevent paid users from being banned. Since banning may result in destructive actions, we want to ensure that this does not happen to our paid users.

References

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MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run GDK in SaaS mode

   export GITLAB_SIMULATE_SAAS=1

2. Add a user as a member to a group with an ultimate license.
3. In the admin area, attempt to ban the user.
4. You should see an error that says you cannot ban paid users.
5. Remove the user from the paid group. You should now be able to ban the user.

Related to #523499 (closed)