Help users apply compatibility mode in the right way

[Legioth]

Spring Boot actively ignores web fragments which means that the flow-server-compatibility-mode dependency isn't automatically enabling compatibility mode. This is mentioned in the documentation but there's still a significant risk that users are struggling because they didn't notice the documentation.

To deal with this, Vaadin could explicitly look for the situation when flow-server-compatibility-mode is present (potentially by adding an additional marker to that package) and in case the property also isn't set, then either explicitly set it or throw an exception at startup.