We missed merging this change, I'm just re-doing it in the right file: https://github.com/hashicorp/terraform/pull/31753

Prevously the cloud backend would only render post-plan run tasks. Now
that pre-plan tasks are in beta, this commit updates the plan phase to
render pre-plan run tasks.  This commit also moves some common code to
the common backend as it will be used by other task stages in the
future.