Centralize PARAM_YEAR_MONTH into RequestParameters

Overall this ends up being nicer.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181782569

java/google/registry/billing/GenerateInvoicesAction.java

@@ -15,6 +15,7 @@
 package google.registry.billing;
 
 import static google.registry.request.Action.Method.POST;
+import static google.registry.request.RequestParameters.PARAM_YEAR_MONTH;
 import static javax.servlet.http.HttpServletResponse.SC_INTERNAL_SERVER_ERROR;
 import static javax.servlet.http.HttpServletResponse.SC_OK;
 
@@ -120,7 +121,7 @@ public class GenerateInvoicesAction implements Runnable {
             .countdownMillis(Duration.standardMinutes(10).getMillis())
             .param(BillingModule.PARAM_JOB_ID, jobId)
             // Need to pass this through to ensure transitive yearMonth dependencies are satisfied.
-            .param(BillingModule.PARAM_YEAR_MONTH, yearMonth.toString());
+            .param(PARAM_YEAR_MONTH, yearMonth.toString());
     QueueFactory.getQueue(BillingModule.BILLING_QUEUE).add(publishTask);
   }
 }