Allow several font sizes for Subhead headings

[camertron]

What are you trying to accomplish?

This PR allows adjusting the (font) size of headings in the Subhead component. A Hubber asked in Slack why Subhead headings in PVC have a font size of 24px, but a font size of 20px in PRC and in Figma. Rather than change the font size and potentially mess with a bunch of existing usages in dotcom, I have chosen to introduce two heading sizes, large (the default), and medium.

Screenshots

Large	Medium

Integration

No changes necessary in production.

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: 3888ac4

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

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Minor

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[lesliecdubs]

Thanks for working on this, @camertron!

Let's make sure to get an approval from someone on Primer Design on this PR before merge.

[jonrohan]

Code changes look good 👍🏻

[langermank]

Suggesting primitives, but not blocking