Migrate plural DNS writers field to being the canonical one

After this point all data is migrated to use the new canonical
plural version, and subsequent code changes can be made that use
multiple writers.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161673486

javatests/google/registry/testing/DatastoreHelper.java

@@ -268,7 +268,7 @@ public class DatastoreHelper {
         // Always set a default premium list. Tests that don't want it can delete it.
         .setPremiumList(persistPremiumList(tld, DEFAULT_PREMIUM_LIST_CONTENTS.get()))
         .setPremiumPricingEngine(StaticPremiumListPricingEngine.NAME)
-        .setDnsWriter(VoidDnsWriter.NAME)
+        .setDnsWriters(ImmutableSet.of(VoidDnsWriter.NAME))
         .build();
   }