Mark MaxMapCountCheck#isSystemCheck as final

This commit marks the method MaxMapCountCheck#isSystemCheck as final as
this method should not be modifiable by inheriting classes (used in
tests).

core/src/main/java/org/elasticsearch/bootstrap/BootstrapCheck.java

@@ -473,7 +473,7 @@ final class BootstrapCheck {
         }
 
         @Override
-        public boolean isSystemCheck() {
+        public final boolean isSystemCheck() {
             return true;
         }