Issue #3534334 by berdir, nicxvan: Convert template_preprocess in form.inc

core/core.services.yml

core/includes/form.inc

core/lib/Drupal/Core/Form/FormPreprocess.php

core/lib/Drupal/Core/Theme/ThemeCommonElements.php

...eld_ui/templates/form-element--new-storage-type.html.twig

core/modules/media/media.module

core/modules/system/templates/checkboxes.html.twig

core/modules/system/templates/details.html.twig

core/modules/system/templates/fieldset.html.twig

core/modules/system/templates/form-element-label.html.twig

core/modules/system/templates/form-element.html.twig

core/modules/system/templates/form.html.twig

core/modules/system/templates/input.html.twig

core/modules/system/templates/radios.html.twig

core/modules/system/templates/select.html.twig

core/modules/system/templates/textarea.html.twig

core/modules/system/templates/vertical-tabs.html.twig

...themes/views_test_theme/templates/input--submit.html.twig

...mami/themes/umami/templates/classy/form/details.html.twig

...ami/themes/umami/templates/classy/form/fieldset.html.twig