Experimental SelectPanel: use radio visual for single selection

[siddharthkp]

• Part of https://github.com/github/primer/issues/3170

Changelog

Before	After

Rollout strategy

• [x] Patch release
• [ ] Minor release
• [ ] Major release; if selected, include a written rollout or migration plan
• [ ] None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation
• [x] Added/updated previews (Storybook)
• [x] Changes are SSR compatible
• [x] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [x] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests): No instances of selectionVariant=single on production