Default postgresql version

[Andreas Enge]

Hello,

I have just pushed this commit
   https://issues.guix.gnu.org/74006
that changes the default postgresql version from 10 (which has a CVE) to
just the postgresql variable (now at 15).

Since as I understood changing postgresql versions requires a (manual?)
update of the database format, I am wondering whether we should
- instead again choose a default value of a specific version?
  or maybe #f and force the user to choose one themselves?
- add a news item to be displayed during "guix pull"?

Andreas