nixos/openssh: Add sntrup761x25519-sha512 kexAlgo
nixos/openssh: Add sntrup761x25519-sha512 kexAlgo

Introduced in OpenSSH 9.0 it became the part of the default kexAlgorithm
selection, visibile in sshd_config(5).

It is also enabled by default in the OpenSSH client, as can be seen from

$ ssh -Q KexAlgorithms

Also clarifies that we use the referenced documents as the lower bound,
given that they haven't been updated for 5-7y.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs