Related posts block: add srcset to thumbnails

[sgomes]

Proposed changes:

In an effort to move away from the devicepx script towards native functionality, this PR adds a srcset to related posts thumbnails (the block, not the module feature). The browser will automatically pick the most appropriate image from the options in the srcset, depending on the screen pixel ratio and the current zoom value.

• Add srcset to the related posts block thumbnails
• Add 1.5x multiplier, because this is an extremely common screen density

This PR complements #31432, by dealing with the related posts block instead of the module feature, this time.

Other information:

• N/A 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

None.

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

No.

Testing instructions:

• Add a "Related Posts" block to your test post, and ensure that it can find at least 3 related posts.
• Enable thumbnails in the block options.
• Visit the post.
• Verify that the thumbnails include a srcset with different pixel ratios (up to 4x, if the source image is large enough).

[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 > Jetpack and enable the update/add-srcset-to-related-posts-block branch.

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

  bin/jetpack-downloader test jetpack update/add-srcset-to-related-posts-block
  

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:

• ✅ Include a description of your PR changes.
  
• ✅ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ 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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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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Once your PR is ready for review, check one last time that all required checks (other than "Required review") appearing at the bottom of this PR are passing or skipped.
Then, add the "[Status] Needs Team Review" label and ask someone from your team review the code. Once reviewed, it can then be merged.
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Jetpack plugin:

• Next scheduled release: August 23, 2023.
• Scheduled code freeze: August 15, 2023.

[sgomes]

Thank you for the review, @jeherve! 🙌