* Replace v0.11 bracket type hints with Terraform v0.12 list expressions
* Use expression syntax instead of interpolated strings, where suggested
* Update Google Cloud tutorial and worker pools documentation
* Define Terraform and plugin version requirements in versions.tf
* Require google ~> 2.5 to support Terraform v0.12
* Require ct ~> 0.3.2 to support Terraform v0.12
If no region is set at the Google provider level, Terraform fails to
create the google_compute_target_pool.workers resource and complains
with "Cannot determine region: set in this resource, or set provider-level 'region' or 'zone'."
This commit fixes the issue by explicitly setting the region for the
google_compute_target_pool.workers resource.
* Background: A managed instance group of workers is used in backend
services for global load balancing (HTTP/HTTPS Ingress) and output
for custom global load balancing use cases
* Add worker instances to a target pool load balancing TCP/UDP
applications (NodePort or proxied). Output as `worker_target_pool`
* Health check for workers with a healthy Ingress controller. Forward
rules (regional) to target pools don't support different external and
internal ports so choosing nodes with Ingress allows proxying as a
workaround
* A target pool is a logical grouping only. It doesn't add costs to
clusters or worker pools
