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

abstract-document/src/test/java/com/iluwatar/abstractdocument/AbstractDocumentTest.java

@@ -25,7 +25,6 @@ package com.iluwatar.abstractdocument;
 
 import org.junit.jupiter.api.Test;
 
-import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;