Remove main landmark from PageLayout.Content.

[radglob]

Reintroducing these changes which were introduced in #3154, but were reverted.

Closes https://github.com/github/primer/issues/1586

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Merge checklist

• Added/updated tests
• Added/updated documentation
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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🦋 Changeset detected

Latest commit: d04beb2

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

This PR includes changesets to release 1 package

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@primer/react	Patch

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[joshblack]

@radglob what does the splash zone of this change look like upstream? Are there any changes we need to make in dotcom before landing this?

[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	104.65 KB (-0.01% 🔽)
dist/browser.umd.js	105.21 KB (-0.01% 🔽)

[radglob]

@radglob what does the splash zone of this change look like upstream? Are there any changes we need to make in dotcom before landing this?

I don't think we have any issues upstream at this point. Armagan had looked into it a bit, and the issue in dotcom was a test that was checking for a main landmark in the react-app element, but it was removed so should no longer be an issue.

[joshblack]

@radglob sweet, sounds great! Opened up: https://github.com/github/github/pull/286317 for the last round of checks but otherwise LGTM! Will leave an approval on the PR once that goes green ✅