Migrate from Terraform v0.12.x to v0.13.x

* Recommend Terraform v0.13.x
* Support automatic install of poseidon's provider plugins
* Update tutorial docs for Terraform v0.13.x
* Add migration guide for Terraform v0.13.x (best-effort)
* Require Terraform v0.12.26+ (migration compatibility)
* Require `terraform-provider-ct` v0.6.1
* Require `terraform-provider-matchbox` v0.4.1
* Require `terraform-provider-digitalocean` v1.20+

Related:

* https://www.hashicorp.com/blog/announcing-hashicorp-terraform-0-13/
* https://www.terraform.io/upgrade-guides/0-13.html
* https://registry.terraform.io/providers/poseidon/ct/latest
* https://registry.terraform.io/providers/poseidon/matchbox/latest

aws/container-linux/kubernetes/bootstrap.tf

@@ -1,6 +1,6 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=3675b3a539efd8d341277f0c03322883f97fd992"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=60540868e070ea95da05fc5a6a5c9a55ce68c58d"
 
   cluster_name          = var.cluster_name
   api_servers           = [format("%s.%s", var.cluster_name, var.dns_zone)]

aws/container-linux/kubernetes/versions.tf

@@ -1,11 +1,15 @@
 # Terraform version and plugin versions
 
 terraform {
-  required_version = "~> 0.12.6"
+  required_version = ">= 0.12.26, < 0.14.0"
   required_providers {
     aws      = ">= 2.23, <= 4.0"
-    ct       = "~> 0.4"
     template = "~> 2.1"
     null     = "~> 2.1"
+
+    ct = {
+      source  = "poseidon/ct"
+      version = "~> 0.6.1"
+    }
   }
 }

aws/container-linux/kubernetes/workers/versions.tf

@@ -1,9 +1,14 @@
+# Terraform version and plugin versions
 
 terraform {
-  required_version = ">= 0.12"
+  required_version = ">= 0.12.26, < 0.14.0"
   required_providers {
     aws      = ">= 2.23, <= 4.0"
-    ct       = "~> 0.4"
     template = "~> 2.1"
+
+    ct = {
+      source  = "poseidon/ct"
+      version = "~> 0.6.1"
+    }
   }
 }