feat(vpn): auto detection of IPv6 support

- `OPENVPN_IPV6` removed
- Affects OpenVPN
- Use the same mechanism for OpenVPN and Wireguard
- Check only once at program start since this is unlikely to change at runtime
- Log if IPv6 is supported
- Remove `IPv6` boolean from settings structs
- Move IPv6 detection as a method on NetLinker

internal/wireguard/constructor_test.go

@@ -55,6 +55,7 @@ func Test_New(t *testing.T) {
 						Mask: net.IPv4Mask(255, 255, 255, 255)},
 					},
 					FirewallMark: 100,
+					IPv6:         ptr(false),
 				},
 			},
 		},