Figure out how to upgrade Postgres

[mockdeep]

The current docker-compose.yml uses Postgres version 9.5 and we need to upgrade to version 16 at the time of writing. We need to figure out an upgrade strategy for our users.

Possible approaches

These need to be tested to see if they can make the full leap from 9.5 to 16, or if we'll need to do them more incrementally. If the full upgrade from 9.5 to 16 cannot be completed in a single step, we may need to create git tags and add documentation to help users gradually upgrade from one version to the next.

• pg_upgrade. The docs say "pg_upgrade supports upgrades from 9.2.X and later to the current major release of PostgreSQL", so we may be able to simply run this when the image boots in order to upgrade users automatically.
• pg_dump. This is more manual and will likely require user interaction to dump the db and reimport it into the most recent version. We will need to confirm whether the dump on version 9.5 is able to be imported into the latest version, or whether we need to incrementally export and reimport into intermediary versions.