ci: update Terraform to 0.15.5

Changelogs:
- https://www.hashicorp.com/blog/announcing-hashicorp-terraform-0-15-general-availability
- https://www.terraform.io/language/upgrade-guides/0-15
- https://github.com/hashicorp/terraform/blob/v0.15/CHANGELOG.md

Fix #1613

[skip ci]

Author: php-coder

infra/terraform/.terraform-version

@@ -1 +1 @@
-0.14.11
+0.15.5

infra/terraform/versions.tf

@@ -1,6 +1,5 @@
 terraform {
-  # @todo #1268 Update Terraform to 0.15.x
-  required_version = ">= 0.14"
+  required_version = ">= 0.15"
 
   required_providers {
     digitalocean = {