Add a feature flag to toggle between `dependencies.json` and GraphQL for the dependency list

Why are we doing this work

$project/-/dependencies.json is being replaced with a new graphql API that was implemented with #390300 (closed). We will be gradually making additions to the GraphQL API until it has full feature parity with dependencies.json. In the meantime, we should add a feature flag to toggle which backend data source is being used by the dependency list. This will allow us to verify changes as we make them.

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• backend Update dependencies controller to use dependencies finder based on the feature flag.

Verification steps