upgrades: freedombox package in bookworm-backports is not installed

The freedombox package in bookworm has priority over the package in bookworm-backports:

$ apt policy freedombox
freedombox:
  Installed: 23.6.2
  Candidate: 23.6.2
  Version table:
     23.13~bpo12+1 100
        100 http://deb.debian.org/debian bookworm-backports/main amd64 Packages
 *** 23.6.2 500
        500 http://mirrors.linode.com/debian bookworm/main amd64 Packages
        100 /var/lib/dpkg/status

This is because, due to a change in the Debian repository, our apt preference file no longer has the intended effect:

$ cat /etc/apt/preferences.d/50freedombox4.pref 
Explanation: This file is managed by FreedomBox, do not edit.
Explanation: Allow carefully selected updates to 'freedombox' from backports.
Package: src:freedombox
Pin: release a=bookworm-backports
Pin-Priority: 500

In the Pin line, a= refers to an archive or suite, such as "stable". a=bullseye-backports worked because the bullseye-backports Release file has "Suite: bullseye-backports". However in the bookworm-backports Release file, it has "Suite: stable-backports" (and "Codename: bookworm-backports"). To prefer the package in bookworm-backports, we need to use n=bookworm-backports in the Pin line now.