Debian Trixie includes OpenSSH 9.8 that has completely removed support for the old and insecure DSA keys (see https://www.debian.org/releases/trixie/release-notes/issues.en.html#openssh-no-longer-supports-dsa-keys).

VyOS 1.6 will almost certainly be based on Trixie and unable to support those, so we should add a deprecation warning to currently supported releases to let people know they need to update their keys.

Since people hardly add new DSA keys, we may want to show a warning on login if the user has DSA keys rather than just a commit warning.