Allow existing notification channels to be specified

The newest v3 gcp-site-monitoring module has the ability to specify existing notification channels by ID, rather than having to create new ones based on email addresses. This ties in nicely with the product factory now creating multiple default notification channels for product teams to consume. We need to move this module to make use of these new features, however it is technically a breaking change as moving to v3 of the site monitoring module will cause all uptime checks to be redeployed at the next Terraform apply - see the releases page for info.