Add required_providers to set minimum versions (#464)

versions.tf

@@ -1,4 +1,10 @@
-
 terraform {
   required_version = ">= 0.12"
+
+  required_providers {
+    aws      = ">= 2.8"
+    local    = ">= 1.2"
+    null     = ">= 2.1"
+    template = ">= 2.1"
+  }
 }