Fix fieldset margins

[Seb33300]

Fixes #7044

After these changes, we should be able to apply the h-100 class to our fieldsets to render them with equal heights.

return [
    FormField::addColumn(6),
    FormField::addFieldset('Fieldset 1')->setCssClass('h-100'),
    // ...
    FormField::addColumn(6),
    FormField::addFieldset('Fieldset 2')->setCssClass('h-100'),
    // ...
    FormField::addColumn(12),
    FormField::addFieldset('Fieldset 3'),
    // ...
]

This was not easy to test, so I hope I didn't miss anything.