Disable code references stats in the features endpoint

[emyller]

It's taking quite a toll on database time, and making demos less pleasant.

This functionality should be restored after #6832 is resolved with a solution that makes code references load milliseconds-cheap in the database.

Acceptance criteria

• Gate code references stats annotation in GET /api/v1/projects/<project_pk>/features/ behind a Flagsmith-on-Flagsmith feature flag
• When the flag is off, return null / 0 for the stats fields — do not change the response schema
• When the flag is on, keep current behaviour
• Default the flag to off