API for Namespace members has correct boolean for user using seat

What does this MR do?

Currently, The api to list the group's members with the parameter show_seat_info=true on Gitlab.com returns true even if they are not belonging to the paid seat within the requested namespace.

This MR should do the following.

If API is being used on the self managed instance

• It should check if the License exists and it excludes guest from active count
  1. If it excludes then if the user is not a guest should return true
  2. Else return true

If the API is being hit on Gitlab.com then

• check if within the namespace the user is billed or not

Conformity

• Changelog entry
• Documentation (if required)
• 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

Closes #207120 (closed)