Upgrade Postgres to 18 (manual step)

[ticoombs]

If we upgrade to v18, we need to change the volume mount locations:

https://github.com/pgautoupgrade/docker-pgautoupgrade?tab=readme-ov-file#error-message-when-mounting-data-to-varlibpostgresqldata-on-postgres-v18

[dessalines]

Good call. I don't think we'll need them to run anything manual, it'll be just changing this line

[ticoombs]

I agree, was mostly for the 1.6.0 release notes about the change (and for myself to remember!). Anyone who has forked us will need to make the change.