Move 'invite_members_new_dropdown_experiment_percentage' under foss [RUN AS-IF-FOSS]

What does this MR do?

Resolves issue where code is in foss, but feature flag yml is in ee

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