chore: update DigitalOcean Terraform Provider to 2.28.1

$ vim versions.tf
$ terraform init
$ terraform plan

Fix #1268

[skip ci]

Author: php-coder

infra/terraform/versions.tf

@@ -1,10 +1,11 @@
 terraform {
+  # @todo #1268 Update Terraform to 0.15.x
   required_version = ">= 0.13"
 
   required_providers {
     digitalocean = {
       source  = "digitalocean/digitalocean"
-      version = "2.22.2"
+      version = "2.28.1"
     }
   }
 }