Update javalin-maven-java example

[dzikoysk]

javalin-maven-java is a bit outdated - there're some compilation errors. It'd be a good idea to sync it again with the core implementation from javalin-gradle-kotlin module.

Errors

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[178,95] annotation is missing element value
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[180,17] illegal start of type
[ERROR] /javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[180,18] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[185,45] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[186,63] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[188,9] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[191,9] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[195,15] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[200,14] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[204,9] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[206,12] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[206,33] class, interface, or enum expected
[ERROR] javalin-openapi/examples/javalin-maven-java/src/main/java/io/javalin/openapi/plugin/test/JavalinTest.java:[310,1] class, interface, or enum expected

Reported in: #240