[Proposal] Deprecate or clarify usage of <legend class="is-required"> for fieldset in forms

**Description:**
While reviewing the Forms / Fieldset required example on the Vanilla Framework website, I noticed the use of for indicating required fields in a radio group. However, I haven't found this pattern in our Figma library, and I haven’t seen it and used in practice, either internally at Canonical or externally.

**Questions:**
- Is this component actively maintained and intended for use?
- If not, should we consider deprecating it from both documentation and code to reduce confusion?

**References:**
Example on the Vanilla site

**Suggestion:**
I propose we review this and decide whether to:
1. Deprecate it officially
2. Update its design and add it to Figma if it's still considered useful

Start by reviewing the Forms / Fieldset required example on the Vanilla Framework website and the referenced Figma library. Determine whether the pattern is maintained and intended for use, then document the decision about deprecation or continued use. Done means the decision and any resulting documentation, code, and design-library updates are aligned.

Written by the indexing model from the issue text.