Move features to core: "Incremental Rollout"

Overview

As part of an effort to open source some currently paid features and move them to our free tier. Incremental rollout has been identified as a feature worth moving down.

Proposal

Incremental Rollout 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.

https://docs.gitlab.com/ee/ci/environments/incremental_rollouts.html