Add subtitle slot to the Primer::Alpha::Overlay::Header and Primer::Alpha::Dialog::Header

[kendallgassner]

What are you trying to accomplish?

I would like to add a link inside of a subtitle. In order to do this I need the subtitle field to be a slot.

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Integration

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• Low risk the change is small, highly observable, and easily rolled back.
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• Fixes axe scan violation - This change fixes an existing axe scan violation.
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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 detected

Latest commit: 44e33d9

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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[camertron]

Would you mind adding the same slot to the Dialog header so the APIs are consistent?

[kendallgassner]

@camertron can you merge this 🙏 . I don't have merge access.