[13.2] Update App Host Events

[afscrome]

Update any eventing docs to account for new helper events added #14119 (e.g. builder.Eventing.OnBeforeStart(...))

• https://aspire.dev/app-host/eventing/#apphost-eventing
• Most uses of Eventing.Subscribe<T> in code snippets / other docs

The signature of the calls is the same, so it should be as simple as a find and replace of Eventing.Subscribe<BeforeStartEvent> with builder.Eventing.OnBeforeStart, although there may be a few examples that need to remain calling the .Subscribe<T> method directly.