Radios element missing "required" attribute

Problem/Motivation

The radios elements that are provided by core do not apply the required attribute to the generated <input> elements. Instead the required attribute is applied to the wrapping <fieldset>.

There are two issues with this:

1. screen readers use required attribute on <input type="radio">. If the attribute is not present on the , then the required state will not be communicated to the screen reader user.
2. the required attribute is not valid when applied to a <fieldset>, thus causing the page to fail W3C validation.

Proposed resolution

1. Remove the required and aria-required attributes from the <fieldset> of Form API radios elements.
2. Add required attribute to the <input type=radio> elements instead.
3. CSS updates, to ensure no visual regression in Seven and Bartik.

Remaining tasks

• Remove required + and aria-required attributes from the <fieldset> element to the <input> element.
• DONE. Add the required="required"<code> attribute to the <code><input type="radio"> elements.
• CSS to avoid visual regression. Asterisk should be after the fieldset legend, not the individual radio buttons.
• Evaluate impact on Classy - Can the CSS go into Classy? - Classy is removed from D10 so may be non issue
• Tests? Appear to have been added

Manual testing tips

We need a required radios element to test this, e.g. add a "favourite colour" field to a content type. Use a field of type "List", mark it required, and use the radios/checkboxes widget under "manage form display".

User interface changes

Markup + CSS changes to

1. Fix invalid HTML
2. Convey required radio buttons correctly to assistive tech.
3. Avoid visual regression.

API changes

None.

Data model changes

None.

Original report by @mfairchild365

This issue is split off from #2950999: Checkboxes element missing "required" attribute. That issue originally addressed radios and checkboxes, but it looks like they need different solutions, so we're using separate issues now.