Earn: Option to cancel subscription without immediate loss of access

### What

When a user's subscriber cancels their subscription to a plan on the site, they [immediately lose access to the subscription content](https://wordpress.com/support/recurring-payments/#how-do-i-cancel-a-payment). Requesting that they instead have the ability to have their access revoked at the expiration of the subscription period (e.g. 1 year from purchase for an annual subscription)

### Why

Currently, if a user cancels a subscription they immediately lose access to the content. Even if they cancel on, for example, month 1 of a 12 month plan.

This can cause issues for subscriber and site owner. Site owners then have to remedy this by refunding original subscription and asking subscriber to resubscribe. This can cost them extra in fees due to the double billing (e.g. 4392007-zd-woothemes).

It seems more consistent with the common subscriber experience to allow the subscription to run its course before access to content is lost.

### How

_No response_

Start by tracing the Earn subscription cancellation flow and how access is revoked. Clarify the expected behavior for monthly and annual plans, then verify that cancellation preserves access through the paid period and avoids requiring a refund and resubscription.

Written by the indexing model from the issue text.