Plans: use the Jetpack Plans package in the Jetpack plugin, and deprecate existing class

[jeherve]

Fixes #31213

Proposed changes:

In this PR, we start using the package in the Jetpack plugin, and deprecate the existing class. No features should be impacted by the change, everything should work just like before.

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

• pdWQjU-4i-p2
• p9dueE-7pi-p2
• 1202726932197503-as-1202726932197515

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

• No

Testing instructions:

Lots of things to test here. I would also recommend tailing the site's logs as you test, to ensure no calls of Jetpack_Plan (now returning deprecation notices) remains.

• Start by connecting your site to WordPress.com.
• Check all the features where files are touched.

[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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• ⚠️ 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
  

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Jetpack plugin:

• Next scheduled release: July 5, 2023.
• Scheduled code freeze: June 26, 2023.

[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 plugin. Once the plugin is active, go to Jetpack > Jetpack Beta > Jetpack and enable the rm/jetpack-plan-current-plan branch.

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

  bin/jetpack-downloader test jetpack rm/jetpack-plan-current-plan
  

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

[anomiex]

Looks ok, diffing trunk projects/plugins/jetpack/class.jetpack-plan.php and projects/packages/plans/src/class-current-plan.php shows minimal differences and some quick poking around with this on wpcom didn't blow up.

Not sure what's going on with all the failing E2E tests.

[anomiex]

Glad I decided to poke at the E2E failures, found a bug.

Also this needs a version bump now.