Change `PGDATA` in 17+ to `/var/lib/postgresql/MAJOR/docker`

[tianon]

This is a pretty large breaking change, which is why this only makes the change in 17+ (which is currently in pre-release stages, and not due for GA until September, and pre-release PGDATA directories are officially not supported on the GA release anyhow).

Concretely, this changes PGDATA to /var/lib/postgresql/MAJOR/docker, which matches the pre-existing convention/standard of the pg_ctlcluster/postgresql-common set of commands, and frankly is what we should've done to begin with, in a classic case of Chesterton's Fence.

This also changes the VOLUME to /var/lib/postgresql, which should be more reasonable, and make the upgrade constraints more obvious.

For any users who have been testing the pre-releases, the simplest way to keep your existing data directory is going to be to add PGDATA=/var/lib/postgresql/data as an environment variable on your container or adjust your bind-mount from /var/lib/postgresql/data to /var/lib/postgresql/17/docker, but the best way is going to be to refactor your host directory such that your data lives at 17/docker inside and you can then mount directly to /var/lib/postgresql (possibly setting PGDATA=/var/lib/postgresql/MAJOR/docker as well, if you want to go overboard on being explicit).