[Callbacks] Merge on_event with on_update, remove MagnitudePruningModifier.leave_enabled

[kylesayrs]

Purpose

• Remove the concept of "on_update" and replace it with on_event, which is triggered only when a modifier has started
• Remove MagnitudePruningModifier.leave_enabled, which was used to simplify recipes but can be replaced by a subsequent ConstantPruningModifier, which much clearer conceptually

Prerequisites

• [Callbacks] Remove MagnitudePruningModifier.leave_enabled #1198

Changes

• Raise a warning when when a modifier receives an event but hasn't been initialized
• Trigger on_event between starting and stopping
• Rename on_update to on_event to remove the concept of "updating" a modifier

The base branch was changed.