Upgrade to version 19 of EKS Terraform module

[jpolchlo]

We're behind on our adoption of the EKS TF module. We've been relying on version 18.x, and the module is up to 19.5.1 at the time of creating this issue. It's not strictly a drop-in replacement, as it requires updating to a much more recent version of the TF AWS module (4.47 from 4.18). This shouldn't prove too difficult, but it may be non-trivial.