YITH WooCommerce Subscription Premium 4.8.0

* New: support for WordPress 5.7
* New: support for WooCommerce 5.1
* Update: YITH plugin framework
* Fix: fixed issues with limited product
* Fix: issue with scheduled subscriptions

New: support for WooCommerce 5.0
Update: YITH plugin framework
Fix: fixed synchronization information inside the product page
Fix: fixed issue on delivery schedules
Dev: added filters 'ywsbs_is_limited', 'ywsbs_needs_flushing'

* Update: Plugin framework
* Fix: Fixed the issue which occurs during the renew order generation if the tax rate changed