Eliminate group templates finder feature flag

What does this MR do?

This MR removes the optimized_groups_with_templates_finder feature flag. It's running on production for more than a week without any issues.

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

Closes #35733 (closed)