Feature flag grade preview renamed to internal for operators

Each FEATURE_<NAME> variable now takes a rollout grade: dev, internal, adoption, prod or off. internal features stay off until a platform operator enables them for an organization, adoption features are opt-in for organization owners and admins, and prod features are on by default.

Upgrade your configuration:

• FEATURE_*=preview becomes internal.
• true becomes adoption to keep organization opt-in, and false becomes off.

Invalid values, including the old ones, disable the feature. The separate DEPLOYMENT_GRADE=preview is unchanged.

See Environment variables. PRs #4115, #4137.