hardening guide: preparing for Debian 13

[perkelix]

Something to remain aware of for the next Debian release:

openssh (1:9.8p1-5) unstable; urgency=medium

  * Add openssh-client-gssapi and openssh-server-gssapi packages; these
    currently just depend on their non-gssapi counterparts, but will become
    different in future.  See
    https://lists.debian.org/debian-devel/2024/04/msg00044.html.

The package description:

 This package provides versions of the ssh client and related programs
 built with support for GSS-API authentication and key exchange, which
 can be used with systems such as Kerberos.
 It is currently an empty package depending on openssh-client, but
 future releases will remove GSS-API support from openssh-client, so
 users who need it should install this package.

Basically, support for GSS variants will become optional and get packaged separately. This will affect the suggested configurations in the hardening guide for Debian 13.