Move features to core: "Feature Flags"

Overview

As part of an effort to open source some currently paid features and move them to our free tier "Feature Flags" have been identified as a feature worth moving down.

Proposal

Feature Flags should be moved to Core. This requires the removal of any licensing checks for the feature which is currently in the Premium tier.

Documentation

Documentation for this feature should be updated to reflect that this is no longer in a paid tier.

• Documentation: https://docs.gitlab.com/ee/user/project/operations/feature_flags.html
• MR/Issue that Implemented the feature: !7433 (merged)