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

### Motivation

The following properties have been deprecated in V24.7, see https://github.com/vaadin/web-components/pull/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.