I had to make only two changes to my project when migrating from 25.2 to 25.3.:
Set version of junit-jupiter explicitly to latest version. Implicit version that came from somewhere didn’t work with browserless tests any more.
Fix a selenium test that searched last row of a Grid by sorting vaadin-grid-cell-contents by slot attribute. Header row slot names are now vaadin-grid-header-cell-content- and footer slots vaadin-grid-footer-cell-content-. Normal cells are still named vaadin-grid-cell-content-
Feel free to add your migration tips here.