Remove aria-required from fieldset to match ARIA spec

I created a MR to address this (using matoeil's code), needs review.

aria-required is not added by Bootstrap5 theme, it's added either by core or by other modules:
Core: > web/core/lib/Drupal/Core/Render/Element/RenderElementBase.php (Drupal 10.5):
line: 145: $element['#attributes']['aria-required'] = 'true';

please raise the issue with core or other modules.