Use typed event listener instead of old deprecated OC_Hooks

[MorrisJobke]

Used in https://github.com/nextcloud/circles/blob/master/lib/AppInfo/Application.php#L136

Please use:

• OCP\Group\Events\GroupDeletedEvent for 'OC_User', 'post_deleteGroup'
• OCP\User\Events\UserDeletedEvent for 'OC_User', 'post_deleteUser'

use OCP\Group\Events\GroupDeletedEvent;
use OCA\Circles\Listener\ GroupDeletedListener;

...
$dispatcher = $this->get(IEventDispatcher::class);
$dispatcher->addServiceListener(GroupDeletedEvent::class, GroupDeletedListener::class);

Ideally this happens in the register() method of the application. (see https://github.com/nextcloud/server/blob/bd6a6cf3bff74dc5690fe032a4203165227f01d2/apps/user_status/lib/AppInfo/Application.php#L70 for an example)

See https://github.com/nextcloud/server/blob/caff1023ea72bb2ea94130e18a2a6e2ccf819e5f/apps/files_sharing/lib/Listener/UserAddedToGroupListener.php for example how the listener can look like.

Those hooks will be dropped in Nextcloud 21 and you can use the new system in all versions starting from Nextcloud 17.