Issue #3620300: Say the booking is confirmed once, not twice

The confirmation page said "Your booking is confirmed." twice, with the booking list between the two.

n_confirm carries stacking: keep, so its confirmation and the bookings list stay on the page. n_await_guarantee, the wait node it forks into, then announced the same sentence again.

Its interaction is removed. It waits for the guarantee window and asks the booker for nothing, and the confirmation it repeated is already on the page and richer. n_cancel_window, the other branch of the same fork, already has no message of its own.

A site with the workflow already installed also needs its active config migrated and a new workflow version published, since config/install only reaches a fresh install and running instances execute a pinned snapshot.