Give user hint to use JDK11

[rladstaetter]

I've created an example project with

 mvn -B archetype:generate -DarchetypeGroupId=com.gluonhq -DarchetypeArtifactId=client-archetype-javafx -DarchetypeVersion=0.0.2 -DgroupId=com.gluonhq -DartifactId=gluon-client-sample -Dversion=1.0.0-SNAPSHOT`

which an example project as expected. When I want to run it with

 mvn javafx:run

it breaks with

 [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project gluon-client-sample: Fatal error compiling: invalid flag: --release -> [Help 1]

The problem is that I'm running JDK8, it would be nice if I got an error message saying 'You should use Java11' for example. One easy way would be to add

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-enforcer-plugin</artifactId>
            <version>3.0.0-M3</version>
            <executions>
                <execution>
                    <id>enforce-jdk</id>
                    <goals>
                        <goal>enforce</goal>
                    </goals>
                    <configuration>
                        <rules>
                            <requireJavaVersion>
                                <version>11</version>
                            </requireJavaVersion>
                        </rules>
                    </configuration>
                </execution>
            </executions>
        </plugin>

to the generated pom.xml.