upstream: Turn off finite field (a.k.a modp) Diffie-Hellman key

upstream: 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@

OpenBSD-Commit-ID: 4e238ad480a33312667cc10ae0eb6393abaec8da