fix: add hypervisor variable and KVM instance type guidance for EKS

[anners]

Closes #25

Summary

• Adds a hypervisor variable (xen or kvm, default xen) to variables.tf
• Updates AMI filter in main.tf to select the correct AMI based on the hypervisor variable (eks-xen-node vs eks-kvm-node)
• Updates terraform.tfvars.example with the hypervisor option, commented KVM instance type example, and list of supported KVM instance families
• Updates cluster_version default to 1.35 in both variables.tf and terraform.tfvars.example