fix: specify minimum required terraform version (>= 1.1)

The module uses `nullable = false` on variables which requires
Terraform 1.1+. Without `required_version` set, users on older
versions would get confusing errors instead of a clear version
constraint message.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -281,6 +281,7 @@ resource "cloudflare_record" "srv" {
 
 | Name | Version |
 |------|---------|
+| <a name="requirement_terraform"></a> [terraform](#requirement\_terraform) | >= 1.1 |
 | <a name="requirement_cloudflare"></a> [cloudflare](#requirement\_cloudflare) | >= 3.0, < 5.0 |
 
 ## Providers

versions.tf

@@ -1,4 +1,6 @@
 terraform {
+  required_version = ">= 1.1"
+
   required_providers {
     cloudflare = {
       source  = "cloudflare/cloudflare"