Postgres 15

Since Postgres 15 was released recently, do you consider adding it in the near future?
`fly pg create --image-ref flyio/postgres:15`

Thanks :)

Start at the `fly pg create --image-ref flyio/postgres:15` entry point and determine how supported Postgres image versions are configured. Done means creating a cluster with the Postgres 15 image works successfully and the relevant behavior is covered by the repository's existing checks.

Written by the indexing model from the issue text.