WooCommerce Group Coupons 2.9.0

* WordPress 5.8 compatible.
* WooCommerce 5.4 compatible.
* Requires at least WooCommerce 5.0.
* Fixed missing variable initialization in Groups coupon settings tab.
* Fixed slashes need to be stripped for group names output.
* Added output escaping where missing.
* Minor code format fixes.
* Updated deprecated links.
* Updated the translation template.
* Updated translations.
* Added translations for Austrian German.
* Added translations for Swiss German.
* Added support for WPML to translate the customized error message.
* Fixed customized error message did not apply translation.

* WordPress 5.8 compatible.
* WooCommerce 5.2 compatible.
* Requires at least WordPress 5.5.
* Requires at least WooCommerce 4.9.

* WordPress 5.7 compatible.
* WooCommerce 5.1 compatible.
* WooCommerce 5.0 compatible.