Do not notify Discord for draft pull requests (#33446)

When I added the `ready_for_review` event in #32344, no notifications
for opened draft PRs were sent due to some other condition. This is not
the case anymore, so we need to exclude draft PRs from triggering a
notification when the workflow is run because of an `opened` event. This
event is still needed because the `ready_for_review` event only fires
when an existing draft PR is converted to a non-draft state. It does not
trigger for pull requests that are opened directly as ready-for-review.