[Spring] Spotbugs static class and default encoding (#2609)

* add static modifier - mvn verify passes

* specify charset, mvn verify works, compiles and clean on spotbugs in my project

* update samples as per contribution guide; mvn integration-test passes

* commit rest of samples

* fix missing newline at EOF

* lots of new newlines

* needed to run mvn clean beforehand...

samples/server/petstore/spring-mvc-j8-async/src/main/java/org/openapitools/api/ApiUtil.java

@@ -16,4 +16,4 @@ public class ApiUtil {
             throw new RuntimeException(e);
         }
     }
-}
+}