Explicitly set terraform version in preparation for the incoming 1.13.0 update

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215806094

java/google/registry/proxy/terraform/modules/common.tf

@@ -1,3 +1,4 @@
 provider "google" {
+  version = "<= 1.12.0"
   project = "${var.proxy_project_name}"
 }