Update GEX fallback processing for OpenSSH

[jtesta]

After over 7 years, the OpenSSH project seems to have accepted my patch to remove the Diffie-Hellman group exchange fallback mechanism! See: https://bugzilla.mindrot.org/show_bug.cgi?id=2793

It sounds like a variation of my patch will go into the next release (v10.0). Assuming it works the way I imagine it, ssh-audit can restrict the current post-processing logic to only OpenSSH v9.9 and lower. Hence, against v10.0 and above, ssh-audit will report the exact results it gets without having to explain any subtleties of the results to the user.

[jtesta]

The patch was committed in openssh/openssh-portable@97eb247.