Add java 11 support for #987 (o-t) (#1051)

* Use java 11

* Use .of
- Replace Arrays.asList with List.of
- Replace HashSet<>(List.of()) with Set.of

* Formatting

mediator/src/test/java/com/iluwatar/mediator/PartyMemberTest.java

@@ -32,7 +32,6 @@ import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 import org.slf4j.LoggerFactory;
 
-import java.util.Arrays;
 import java.util.Collection;
 import java.util.LinkedList;
 import java.util.List;
@@ -50,7 +49,7 @@ import static org.mockito.Mockito.verify;
 public class PartyMemberTest {
 
   static Collection<Supplier<PartyMember>[]> dataProvider() {
-    return Arrays.asList(
+    return List.of(
             new Supplier[]{Hobbit::new},
             new Supplier[]{Hunter::new},
             new Supplier[]{Rogue::new},