Specify the requirements for building Kogito Runtimes

The README.md document does not specify the requirements: Maven and Java.

The problem is that we're using the plugin using the release argument:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.0</version>
    <configuration>
        <release>11</release>
    </configuration>
</plugin>

This argument relies on a new JVM option released on JDK 9 and since we only support LTS versions, we should specify the requirement JDK 11+.