test: lock down max terraform version to <1.6 (#604)

Co-authored-by: Andrej <[email protected]>
Co-authored-by: Conall Ó Cofaigh <[email protected]>

Author: 3 people

tests/resources/version.tf

@@ -1,5 +1,5 @@
 terraform {
-  required_version = ">= 1.3.0"
+  required_version = ">= 1.3.0, <1.6.0"
   required_providers {
     tls = {
       source  = "hashicorp/tls"