Support pgvector pg18

[IlyaSemenov]

Currently, pgvector pg18 wouldn't start:

# dokku postgres:create test-pgvector  --image "pgvector/pgvector" --image-version "pg18"
...
       Error: in 18+, these Docker images are configured to store database data in a
              format which is compatible with "pg_ctlcluster" (specifically, using
              major-version-specific directory names).  This better reflects how
              PostgreSQL itself works, and how upgrades are to be performed.
              See also https://github.com/docker-library/postgres/pull/1259
              Counter to that, there appears to be PostgreSQL data in:
                /var/lib/postgresql/data (unused mount/volume)
              This is usually the result of upgrading the Docker image without
              upgrading the underlying database using "pg_upgrade" (which requires both
              versions).
              The suggested container configuration for 18+ is to place a single mount
              at /var/lib/postgresql which will then place PostgreSQL data in a
              subdirectory, allowing usage of "pg_upgrade --link" without mount point
              boundary issues.
              See https://github.com/docker-library/postgres/issues/37 for a (long)
              discussion around this process, and suggestions for how to do so.

I know this was fixed for vanilla Postgres (#365), but apparently pgvector does something differently.

The current workaround I'm using is to add --custom-env "PGDATA=/var/lib/postgresql/data", but I suppose it deserves a proper fix in the plugin.