maven.3.8.x: fixes required for pom.xml

If using Maven.3.8x then 

```
<maven.compiler.source>17</maven.compiler.source>
<maven.compiler.target>17</maven.compiler.target>
```

have to be added to the properties as well as a version must be specified for the `maven-war-plugin`. Version 3.3.1 seems to do the trick.

Alternatively, the README could state that maven.3.9.x is required.
