Change the module to require only the APCu extension

To make the module compatible with APCu, also the code in apc_requirements() must be changed. apc_requirements() cannot assume the APCu Backwards Compatibility Module extension is installed. That could be true most of the times, but it is not necessarily true.

Also, since the APC extension is no longer maintained and has been superseded, the code can simply rely on the apcu_*() functions.

The last stable APC version has been releases on May, 2011, five years and a half before this issue was open.
Hopefully, no Drupal 7 site is still running on PHP 5. They are more probably running on PHP 7 or 8, with the APCu extension.

Automatically closed - issue fixed for 2 weeks with no activity.