Rename `SchedulerNotificationsConstraint.notificationContent` to `updateNotificationContent`

[lukaskollmer]

Problem

The current name (notificationContent) makes it seem like the function is responsible for providing the notification content, which is not true.

Solution

Calling it updateNotificationContent instead would signal that the point of the function is to give the app an opportunity to modify the notification content, should it want to.

We should include this change in the next major release of the package.

Additional context

No response

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