Insights is now GA

What does this MR do?

Document the fast that Insights in now Generally Available as of %12.0.

The MR that removes the feature flag is https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14067.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry (in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14067)
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content