Prerelease: backport Boost 4.6.3 changelogs to trunk

[LiamSarsfield]

Proposed changes:

Backports changelog/readme/version updates from the Boost 4.6.3 release. Includes the My Jetpack SEO opt-in fatal fix (#49988 / JETPACK-1775).

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

N/a

Does this pull request change what data or activity we track or use?

No

Testing instructions:

Proofread changes.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the prerelease branch.
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bin/jetpack-downloader test jetpack prerelease

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[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• 🔴 Add a "[Status]" label (In Progress, Needs Review, ...).
  
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• ✅ Add changelog entries to affected projects
  

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[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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