[FIX] web: form_compiler, classes on FormLabel in a group

Before this commit, when compiling a form group, the scope class is
added to the elements of the outer group. It was added on the element
class attribute. The issue is that FormLabel don't have a class
attribute but a className attribute. So the class wasn't shown on the
DOM.

Now, the class is added on the FormLabel className attribute and shown
in the DOM.

closes odoo/odoo#105184

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
Co-authored-by: Lucas Perais (lpe) <lpe@odoo.com>