Update PostgreSQL to newer version

[StevenMaude]

The PostgreSQL version in use is apparently 13, judging from the Docker configuration, as introduced here back in 2021.

It's not been updated to a new major version since, as far as I can tell, although I can't check what Digital Ocean is actually running.

The end of life for PostgreSQL 13 is a year today — 2025-12-23 — so we should update before then.

This will require work along the lines of:

• some local testing to see if there any issues with upgrading the database
• fixing any issues if there are
• upgrading the PostgreSQL version in docker-compose.yml
• backing up the database immediately prior to upgrade (this may happen automatically on upgrade)
• upgrading the database via Digital Ocean, which apparently can happen with no downtime
• confirming the upgrade has succeeded
• developers working on job-server upgrading their local setups, if they're not using Docker to run the database

One potential issue depending on how access to the current database is controlled; for password authentication, newer versions of PostgreSQL enforce a newer password authentication scheme than older versions. It may be that any such database credentials need reconfiguring before or after the PostgreSQL upgrade.