Add Flow/Fusion vs Starter validation into Flow Validation

[mshabarov]

Description of the bug / feature

Prerequisites: to spot earlier a possible regressions in Flow/Fusion preventing Spring-based applications to work as expected. Validation with starter is now possible during Flow release process, but this is too late for finding the critical issues.

Proposed solution:

1. Run the Spring Starter's tests given in https://github.com/vaadin/skeleton-starter-flow-spring within the Flow validation.
2. This shall give a vision of whether last changes in Flow/Fusion do not break the basic Spring-based application.
3. Here are the old muted builds with starters https://bender.vaadin.com/project.html?projectId=Flow_StarterTests&tab=projectOverview for reference.
4. Preferably run this test in parallel with the add-ons validations to safe the time.
5. This ticket is an addition to https://github.com/vaadin/flow/issues/11890.

[denis-anisimov]

Let's don't use Flow validation build task for this. Let's create a build task for Spring skeleton starter and make a trigger for it: run it once the Flow SNAPSHOT is available.