Maintenance (mostly docs & workflow for CodeQL)
- Ubuntu 18.04->20.04 makes Python 3.6 -> 3.8 and PostgreSQL 10 -> 12
- Removed unmaintained macOS instructions
- Updated RAM and storage requirements
- Removed obsolete plugins from docs
- Fixed relative submodule reference
- Added CodeQL workflow with Python 3.8
- some minor workflow fixes / improvements