Performance: use preconnect over dns-prefetch

[kraftbj]

Fixes #10780

Proposed changes:

• For domains that are virtually certain to be used (e.g. the CDN domains when CDNs are enabled), use preconnect instead of dns-prefetch.
• When there is uncertanity, remain with dns-prefetch to avoid extra, possibly unneeded, work.

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?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

n/a

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

No.

Testing instructions:

• Verify that the resource hint in the header shows preconnect.

[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 WordPress.com Simple 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, and enable the update/resource-hints branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/resource-hints
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[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:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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Follow this PR Review Process:

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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen semi-continuously (PCYsg-Jjm-p2).
• WoA releases happen weekly.
• Releases to self-hosted sites happen monthly. The next release is scheduled for none scheduled (scheduled code freeze on undefined).

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

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Boost plugin:

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[haqadn]

Looks good! 👍🏼