Turn off finite field (a.k.a modp) Diffie-Hellman key exchange in sshd

Turn off finite field (a.k.a modp) Diffie-Hellman key exchange in sshd
by default. Specifically, this removes the diffie-hellman-group* and
diffie-hellman-group-exchange-* methods. The client is unchanged and
continues to support these methods by default.

Finite field Diffie Hellman is slow and computationally expensive for
the same security level as Elliptic Curve DH or PQ key agreement while
offering no redeeming advantages.

ECDH has been specified for the SSH protocol for 15 years and some
form of ECDH has been the default key exchange in OpenSSH for the last
14 years.

ok markus@