login: T7839: add deprecation warning for DSS public-key usage

OpenSSH in Debian Trixie has removed support for ssh-dss (DSA) keys, which will
prevent users with such keys from logging in after upgrade. To avoid lockouts,
add a loud deprecation warning when users log in using a DSA key. This warning
advises affected users to replace their keys with a supported algorithm (e.g.,
ed25519 or RSA) before the upgrade.

Deprecation warning will be displayed during "commit" but also as MOTD to inform
on this issue during every login.

    DEPRECATION WARNING: Support for SSH-DSA keys is deprecated and will
    be removed in VyOS 1.6. Please update affected keys to a supported
    algorithm (e.g., RSA, ECDSA or ED25519) to avoid authentication
    failures after the upgrade. The following users are using SSH-DSS keys
    for authentication.

    User "vyos" with deprecated public-key named: foo

0 files changed, 0 insertions, 0 deletions