[1.0.1] Replace fielset__content with fieldset__element system (1.8 changelog)

Problem/Motivation

Since DSFR v1.8, the form/fieldset structure is different : fieldset__content is deprecated and should be replaced by a fieldset__element wrapper for each form element.
https://github.com/GouvernementFR/dsfr/releases/tag/v1.8.0

This change seems a bit tricky to implement, because each form element should "know" if it's a child of a fieldset to add this fieldset__element wrapper on itself.
Currently the fieldset__content wraps all the fieldset children, so it's not the same system.