Upgrade PostgreSQL to stable/postgresql 7.7.3

Closes #1535 (closed)

1. Update to using stable/postgresql 7.7.3, major version bump
2. Provide documentation: updates regarding secrets, steps to migrate
3. Add script to provide data upgrade path (not that you should have been using our PostgreSQL in production before now!)
4. Add check to upgrade-check Helm hook to attempt to prevent upgrades in a broken state, warn of need to perform steps (from documentation above)