Unsupported diffie-hellman-group-exchange-sha256

[steberk]

Hello,

With the latest 1.0.216 I cannot connect to older network devices.
They supports only the diffie-hellman-group-exchange-sha256 key exchange method.
The new SSH backend does not support this (I could not find in the supported list).

Please implement this method in the SSH backend or make possible to select between the new and the old backend for each profile.

[Eugeny]

Tracking in Eugeny/russh#436