Update developer guidelines for major PG version updates

Overview

1. As we refine our long-term roadmap for supported PostgreSQL version, we need to make some updates to the developer page to outline the plan going forward.

2. Capture the steps and test plans that were created in issues linked to &2184 (closed) so we have a reference for future additions and removals

3. Document any learnings from the PG 11 and PG 12 updates and testing