Undefined function apc_clear_cache() in PHP 7

The idea of 📌 Change the module to require only the APCu extension Needs work is to change this module to require only the APCu extension, since the APC extension has been superseded.
It does not mean this cannot be handled with multiple issues.

Since there is already a patch provided in this issue, I will use the patch provided here to create a merge request.

I had to slightly change the used code, since apcu_clear_cache(), like apc_clear_cache(), always return TRUE. We can only assume the cache has been cleared and show that message to users.

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