Feature Flags: Add introduced by url

What does this MR do?

group_level_integrations feature flag was introduced by !27557 (merged).

We were missing this information in !42454 (merged).

https://docs.gitlab.com/ee/development/feature_flags/development.html#feature-flag-definition-and-validation

Related to #241771 (closed), #251080 (closed), and #238575 (closed)