Jetpack AI: user is not redirected back to post after upgrade

The behavior created by https://github.com/Automattic/jetpack/pull/35263 is not working anymore.

Steps to reproduce:

- go to the block editor, add some content and save the draft
- go to the Jetpack sidebar, look for the AI Assistant section and click the upgrade button
- follow the upgrade process
- after buying it, notice the page you get redirected to

Expected behavior:

- user is redirected back to the editor, with the correct post loaded

Current behavior:

- user is redirected back to My Jetpack page

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Some details about the redirect implementation:

- to implement the redirect, PR #35263 created a parameter called `redirect_to_referrer` that, when present, would indicate to My Jetpack that the `redirect_to` URL provided on the checkout flow should be the referrer URL (for out case, the URL to edit the post the user was editing)
- it's possible that the behavior of looking to `redirect_to_referrer` and deciding which redirect URL to include got lost on recent changes