Investigate using of create_before_destroy option for node pools

In https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/issues/31#note_599611 we had to perform an emergency upgrade of a node pool. In testing it was noted that terraform destroys the existing node pool before creating the replacement.

Investigate using the create_before_destroy lifecycle meta-argument on the node pool resource to ensure that a new node pool is created before the old one is destroyed. This allows for workload migration.