Change access modifier of execute method. (SeleniumHQ#8344)

We have extended RemoteWebDriver and added more capabilities by extending RemoteWebDriver. We have overridden below methods.

```
protected Response execute(String driverCommand, Map<String, ?> parameters) {}
protected Response execute(String command) {}
```

Now these method has been replaced by `CommandPayload` arguments, these need to be a protected method to support override. Earlier it was support in selenium version 2 and 3.

java/client/src/org/openqa/selenium/remote/RemoteWebDriver.java

@@ -569,7 +569,7 @@ public void setLogLevel(Level level) {
     this.level = level;
   }
 
-  Response execute(CommandPayload payload) {
+  protected Response execute(CommandPayload payload) {
     Command command = new Command(sessionId, payload);
     Response response;