Update documentation for MPR configuration with Vaadin 7 + Vaadin 23

[mcollovati]

The documentation for configuring project to run MPR for Vaadin 7 + Vaadin 23 seems to be missing. The current page references Vaadin 14. For Vaadin 23 it is also missing important configurations to make widgetset compilation work with Java 11 (required by Vaadin 23).

MPR 6.0.0 release notes mentions changes that should be applied, but having a full example on documentation may be better.

For example, from the release notes I didn't immediately get that I have to add the gwt-maven-plugin and remove compile goal from vaadin-maven-plugin (v7)

[mcollovati]

Additional notes:

• dependencies mentioned in the MPR release notes should have provided scope
• com.google.gwt:gwt-dev should have an exclusion for commons-io:commons-io otherwise application fails to startup due to java.lang.NoSuchMethodError: 'void org.apache.commons.io.FileUtils.forceMkdirParent(java.io.File)