chore(Label): Remove CSS modules feature flag from Label component

[jonrohan]

This removes the CSS modules feature flag from the Label component that was first introduced 4 weeks ago https://github.com/primer/react/pull/4893

Changelog

Changed

Label is styled by CSS modules.

Rollout strategy

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

Merge checklist

• [x] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Storybook)
• [x] Changes are SSR compatible
• [x] Tested in Chrome
• [x] Tested in Firefox
• [x] Tested in Safari
• [x] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)