Expose user's plan only on EE

What does this MR do and why?

Partially resolves #434036 (comment 1701858870).

A user can only have a GitLab subscription on EE.

This MR updates AbuseReportDetailsEntity to expose a user's plan when on EE only. By doing this we also cleanup specs that unnecessarily use if: Gitlab.ee? in CE specs.

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