Automate WooCommerce 6.0.21

* Dev - Remove local subscription helper.
* Dev - Use `merge-trunk-develop-pr` action.
* Fix - Email actions not imported with WP Importer.
* Update - Bump minimum PHP version to 7.3

* Break - Remove deprecated Events code and automatewoo_events database table.
* Dev - Add Unit Tests for Action Scheduler Jobs.
* Dev - Automate merging trunk to develop after a release.
* Dev - Deprecate Background Process Base Class.
* Dev - Deprecate Cron Workers' usage in StartOnHook based Jobs.
* Dev - Remove deprecated Background Proccess Classes.
* Dev - Unit Tests for Action Scheduler Jobs.
* Fix - Prevent PHP 8.2 deprecation messages.,* Tweak - Add WooCommerce 6.0.0 to Travis test matrix (#1077)
* Tweak - Bump "WC tested up to" to version 6.0 (#1079)

* Add - Activate or Update notification type.
* Add - Inbox notification for Mailchimp API key failure.
* Add - Inbox notification for failed system checks.
* Add - Inbox notification when site is running minimum version of PHP supported by AW.
* Add - Inbox notifications for Add-ons.
* Add - Inbox notifications.
* Add - Migrate existing Welcome Note to Notifications.
* Add - Notifications for remaining integrations.
* Add - Unit tests for NotificationsInitializer.
* Dev - Update Sensei Integration Unit Tests.
* Fix - Avoid error notice on AW installation.
* Fix - Date input pattern.
* Fix - Datepicker initialization in Workflow.
* Fix - Fix Sensei Quiz Rules not working when using Sensei v4.16.0 .
* Fix - Revert trigger when using any method to close incompatible actions modal.
* Tweak - Track currency for conversions.