fix(openvpn/extract): restrict custom openvpn config protocol to tcp or udp internally

- Fix #3179
- I believe specifying tcp4, tcp6 or tcp-client does not change anything versus tcp + remote ip address
- I believe specifying udp4 or udp6 does not change anything versus tcp + remote ip address
- Simplify firewall code to not account for tcp-client etc.

internal/firewall/iptables/iptables.go

@@ -151,9 +151,6 @@ func (c *Config) AcceptOutputTrafficToVPN(ctx context.Context,
 	defaultInterface string, connection models.Connection, remove bool,
 ) error {
 	protocol := connection.Protocol
-	if protocol == "tcp-client" {
-		protocol = "tcp"
-	}
 	instruction := fmt.Sprintf("%s OUTPUT -d %s -o %s -p %s -m %s --dport %d -j ACCEPT",
 		appendOrDelete(remove), connection.IP, defaultInterface, protocol,
 		protocol, connection.Port)