chore: update Terraform to 0.13.7

$ vim .terraform.version
$ tfenv install
$ terraform 0.13upgrade
$ vim my-stamps.tf
$ vim versions.tf
$ git add versions.tf
$ terraform state replace-provider -- -/digitalocean digitalocean/digitalocean
$ terraform init
$ terraform plan

"terraform state replace-provider" is required to prevent terraform to download the lastest
version and use the one that is specified. See hashicorp/terraform#25819

Changelogs:
- https://developer.hashicorp.com/terraform/language/v1.1.x/upgrade-guides/0-13
- https://github.com/hashicorp/terraform/blob/v0.13/CHANGELOG.md

Part of #1268

[skip ci]

Author: php-coder

infra/terraform/.terraform-version

@@ -1 +1 @@
-0.12.31
+0.13.7

infra/terraform/my-stamps.tf

@@ -2,17 +2,12 @@
 # @todo #1000 Terraform: add UptimeRobot
 # @todo #1000 Terraform: add Mailgun
 
-terraform {
-  required_version = ">= 0.12"
-}
-
 variable "do_token" {
 }
 
 # Digital Ocean provider docs: https://www.terraform.io/docs/providers/do/index.html
 provider "digitalocean" {
   token   = var.do_token
-  version = "2.22.2"
 }
 
 # Droplet docs: https://www.terraform.io/docs/providers/do/r/droplet.html

infra/terraform/versions.tf

@@ -0,0 +1,10 @@
+terraform {
+  required_version = ">= 0.13"
+
+  required_providers {
+    digitalocean = {
+      source  = "digitalocean/digitalocean"
+      version = "2.22.2"
+    }
+  }
+}