Remove deprecated underline prop from Link component

[liuliu-dev]

Closes #5301

Removed deprecated underline prop from Link component.

Breaking Change: There are a few places including github-ui are still using Link with underline prop: https://primer-query.githubapp.com/?query=Link+attribute%3A%22underline%22+value%3A%22true%22

Migration: Use the inline prop instead to control underline behavior for links.

Rollout strategy

• Major release; if selected, include a written rollout or migration plan

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: 7698bc2

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/styled-react	Major

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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!