Migrate to Vaadin Flow v21

[ddecouteau]

Currently at release level 14. Vaadin v21 for Flow simplifies spring security.

Spring Security helpers for Vaadin Flow
With improved helpers in vaadin-flow and vaadin-spring, we should make it easier, faster, and safer for Vaadin developers to utilize Spring security in their web apps.
RouteScope for Flow Spring integration
With this feature, you can share beans/component-state inside a route between different UI subparts/fragments, So that you can use DI to inject the beans to the route classes and control when state is shared and when cleaned up
It is possible to keep your RouteScoped beans to be the same after refresh (together with PreserveOnRefresh)
Make it easier for migrating your Vaadin 8 app to Latest