Document removal of DSA key support from OpenSSH
As explained in https://lists.mindrot.org/pipermail/openssh-unix-announce/2024-January/000156.html and https://www.openssh.com/releasenotes.html#9.8p1, OpenSSH 9.8p1 disables DSA support by default at compile-time. I intend to follow this when packaging 9.8p1 for Debian.
Some people may still be using DSA keys, either necessarily for connecting to some old devices, or through inattention. Offer them some advice.