[CSS Module Migration] Remove feature flag for Underline Panel

[jamieshark]

• Towards https://github.com/github/primer/issues/4308

This PR removes the CSS modules feature flag from the FormControl component. The component UnderlinePanel is used 0 times in dotcom.

Changelog

New

Changed

Removed

Feature flag for Underline Panel and associated components

Rollout strategy

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 0bad285

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	105.54 KB (-0.46% 🔽)
packages/react/dist/browser.umd.js	105.82 KB (-0.52% 🔽)

[francinelucca]

LGTM barring integration failures

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/370942

[primer-integration]

🟢 golden-jobs completed with status success.