[form-layout] Remove deprecated form-item custom CSS properties in V25

[web-padawan]

Motivation

The following properties have been deprecated in V24.7, see #8691

• --vaadin-form-item-label-width
• --vaadin-form-item-label-spacing
• --vaadin-form-item-row-spacing

Proposed solution

Remove deprecated custom CSS properties in V25, update Lumo visual tests to use new properties.