Check default branches in all starters/demos/skeletons to be v25

[manolo]

Describe your motivation

There are some projects that could be wrong e.g.
https://github.com/vaadin/base-starter-spring-gradle
https://github.com/vaadin/base-starter-gradle

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response