Update Watch events to include all events

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In discussing renaming the "Watch" notification level to "All", it was flagged that several events are currently missing in the watch notification level (also highlighted in this docs issue):

FYI on Watch/All: @engwan found out that the following events are disabled for Watchers:

• push_to_merge_request
• issue_due
• success_pipeline
• approver

We should include these events in the "Watch" level, as that level is meant to include notifications for any event. We should also make this change before updating the level name to "All".