Add a new error modal for billable members

What does this MR do?

When viewing the seat usage / billable members list, we display members who are indirect members via an invited group.

This can cause confusion for our customers because those members cannot be removed directly due to the nature of their association with the group (i.e. the invited group would need to be removed as a whole).

To address this, two things are going to change:

1. adds a new badge alongside applicable invited group billable members to identify them as such (separate MR: !61366 (merged))
2. This MR: Display an appropriate error when trying to remove those members

Refs #325412 (closed)

Screenshots (strongly suggested)

Testing

The simplest way to test this modal would be to:

1. Modify this method to simply return false
2. View the seat usage page for any of your local groups, e.g. http://localhost:3000/groups/my-group/-/seat_usage
3. Attempt to remove any member (removable should now be false so modal should display)

To test it with a real scenario:

1. Invite a group (Group B) with members into another group (Group A)
2. View Group A's seat usage page, e.g. http://localhost:3000/groups/group-a/-/seat_usage
3. Attempt to remove the member from Group B and view the modal

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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