Add alert for exceeding new user signups cap

What does this MR do?

Adds a new alert that will be shown when an EE instance is reached its active user cap.

Additional notes:

• There is no backend implementation for this feature yet.
• This feature is behind a feature flag admin_new_user_signups_cap

Closes #244493 (closed)

Screenshots (strongly suggested)

Desktop

Mobile

Does this MR meet the acceptance criteria?

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