Increase width of datetime widget

[core23]

Subject

I am targeting this branch, because this is a patch.

Changelog

### Changed
- Increase `datetime_widget` width to 6 when using separated field inputs for date and time.

Given the following form type, the default width is far to small to display the date widget.

            ->add('begin', DateTimeType::class, [
                'label'        => 'form.label_date_begin',
                'required'     => true,
                'row_attr'     => [
                    'class' => 'col-md-6',
                ],
                'date_widget'   => 'single_text',
                'time_widget'   => 'choice',
            ])
            ->add('end', DateTimeType::class, [
                'label'        => 'form.label_date_begin',
                'required'     => true,
                'row_attr'     => [
                    'class' => 'col-md-6',
                ],
                'date_widget'   => 'single_text',
                'time_widget'   => 'choice',
            ])