Bring Product Analytics back to a closed beta state

Problem to solve

With Product Analytics being deprioritised for the time being, we want to reduce the overall maintenance overheads, and give us time to review the current implementation to see where we can improve the overall product from UI to infra.

Proposed solution

1. Change the existing product_analytics_admin_settings feature flag to default to off
2. Add a feature flag to disable Product Analytics by default
3. Update devkit E2E tests.
4. Update dev docs.