set java level to 1.6 to prevent error with annotations with default ver...

When building the war using maven, the following error appears
 ```
<path to project>/hello-java-webapp_runner/src/main/java/org/example/HelloServlet.java:[11,1] annotations are not supported in -source 1.3
(use -source 5 or higher to enable annotations)
@WebServlet(name = "HelloServlet", urlPatterns = {"/hello"})
 ```
To solve this issue, the maven compiler plugin must be explicitly set to the java version 1.5 or higher.