[terraform] update nonces table configuration

Summary: add nonces table to prod, enable TTL

Test Plan: need to test this on staging. `terraform plan` gave me some issues.

Reviewers: bartek, jon

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8832

services/terraform/modules/shared/dynamodb.tf

@@ -230,9 +230,6 @@ resource "aws_dynamodb_table" "identity-tokens" {
 }
 
 resource "aws_dynamodb_table" "identity-nonces" {
-  # This table doesnt exist in prod
-  count = var.is_dev ? 1 : 0
-
   name         = "identity-nonces"
   hash_key     = "nonce"
   billing_mode = "PAY_PER_REQUEST"
@@ -241,6 +238,11 @@ resource "aws_dynamodb_table" "identity-nonces" {
     name = "nonce"
     type = "S"
   }
+
+  ttl {
+    attribute_name = "expirationTimeUnix"
+    enabled        = true
+  }
 }
 
 resource "aws_dynamodb_table" "identity-reserved-usernames" {