(chore) Disable ssl connection to FE by default for compatibility reason

Older MySQL client (< 5.7.28) will try to connect to server with tls1.1,
which is insecure and is not supported by Doris FE. The connection will
fail.

We disable ssl connection support on Doris FE to keep the users' application
unaffected. To enable ssl support explicitly, just put
the following to fe.conf
```
enable_ssl = true
```

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1829,7 +1829,7 @@ public class Config extends ConfigBase {
      * If set to ture, doris will establish an encrypted channel based on the SSL protocol with mysql.
      */
     @ConfField(mutable = false, masterOnly = false, expType = ExperimentalType.EXPERIMENTAL)
-    public static boolean enable_ssl = true;
+    public static boolean enable_ssl = false;
 
     /**
      * If set to ture, ssl connection needs to authenticate client's certificate.