Remove the WordPress Popular Posts classic widget

[cabrerahector]

The "classic" widget has been marked as deprecated since version 6.1.2 (see #362). By the time I start working on this people using the classic widget should have had enough time to migrate to the WordPress Popular Posts block and/or the [wpp] shortcode.

To do:

• [x] Update classic widget copys (eg. PSA, Usage, etc.) on readme.(txt|md)

[cabrerahector]

Decided on a less abrupt approach: instead of completely deleting the widget from the codebase, the widget will now instead display a copy letting users know that it's been removed. No posts will be listed whatsoever. This will be the final push to get people to migrate to either the block or the shortcode.

Issue #381 will take care of removing everything related to the widget from the codebase on a follow-up release, maybe 7.1.0.

[cabrerahector]

Alternatively, we could also just remove the widget form and use $instance to build the popular posts list on the front-end. User would lose the ability to update the widget but in return their list may continue to render on screen which sounds like a fair compromise.

To assist with the migration process, the form could be replaced with a text representation of the shortcode maybe?