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.

server/core/config.cc

@@ -282,6 +282,7 @@ static const MXS_ENUM_VALUE ssl_version_values[] =
     {"TLSv10", SERVICE_TLS10      },
     {"TLSv11", SERVICE_TLS11      },
     {"TLSv12", SERVICE_TLS12      },
+    {"TLSv13", SERVICE_TLS13      },
     {NULL}
 };