Notifier

Addresses #9:

com.apple.notificationcenterui2 has been the bundle ID of Notification Centre up to BS B9.. now it’s com.apple.notificationcenteru, which is the same as any macOS before.

So can revert the changes for this and the issue with the if #available API and BS is now moot as not needed for BS checking anymore.

As such, Big Sur b9+ supported