Return support for diffie-hellman-group14-sha1

It seems that a lot of users rely on this, let's return it and
deprecated later to make the migration less disruptive

config.yml.example

@@ -89,7 +89,7 @@ sshd:
   # Specifies the available message authentication code algorithms that are used for protecting data integrity
   macs: [hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com, hmac-sha2-256, hmac-sha2-512, hmac-sha1]
   # Specifies the available Key Exchange algorithms
-  kex_algorithms: [curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group14-sha256]
+  kex_algorithms: [curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group14-sha256, diffie-hellman-group14-sha1]
   # Specified the ciphers allowed
   ciphers: [aes128-gcm@openssh.com, chacha20-poly1305@openssh.com, aes256-gcm@openssh.com, aes128-ctr, aes192-ctr,aes256-ctr]
   # SSH host key files.

internal/sshd/server_config.go

@@ -32,6 +32,7 @@ var (
 		"ecdh-sha2-nistp384",
 		"ecdh-sha2-nistp521",
 		"diffie-hellman-group14-sha256",
+		"diffie-hellman-group14-sha1",
 	}
 )