MXS-2762: Add ssl_version=TLSv13

Added new ssl_version value for TLSv1.3. This allows the list of accepted
protocol versions to be limited to all supported protocols. Previously
TLSv1.3 was only available with ssl_version=MAX.

Also fixed the enum value serialization to use a lowercase v. This causes
them to have the same value as the one used in the enum.

include/maxscale/ssl.h

@@ -34,6 +34,7 @@ typedef enum ssl_method_type
     SERVICE_TLS10,
     SERVICE_TLS11,
     SERVICE_TLS12,
+    SERVICE_TLS13,
     SERVICE_SSL_MAX,
     SERVICE_TLS_MAX,
     SERVICE_SSL_TLS_MAX,