Admin page component: add isolation style for wp-ui popovers

[simison]

Now that we're using @wordpress/ui in all headers of Admin UI as well as in other places of Jetpack UI, let's set up popovers properly as instructed in the package readme.

Proposed changes

• Adds isolation styles to help portaled popovers from @wordpress/ui to appear correctly.

See https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/isolation

See setup:

Also, to ensure that portaled popovers appear correctly, add these isolation styles to your application's layout root element:

.root {
 isolation: isolate;
}

No before/after since we're not using popovers yet from @wordpress/ui, but it's easy to miss this setup rule for anyone later adding them.

Other information

Related product discussion/links

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

Testing instructions

• Jetpack pages continue to work

[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 update/page-isolation-styles branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/page-isolation-styles

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/page-isolation-styles

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]

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Code Coverage Summary

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Please fix the tests, or re-run the Code coverage job if it was something being flaky.

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