Merge DomainResource into DomainBase

This eliminates the use of Objectify polymorphism for EPP resources entirely
(yay!), which makes the Registry 3.0 database migration easier.

It is unfortunate that the naming parallelism of EppResources is lost between
ContactResource, HostResource, and DomainResource, but the actual type as far as
Datastore was concerned was DomainBase all along, and it would be a much more
substantial data migration to allow us to continue using the class name
DomainResource now that we're no longer using Objectify polymorphism. This
simply isn't worth it.

This also removes the polymorphic Datastore indexes (which will no longer
function as of this change). The non-polymorphic replacement indexes were added
in []

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=230930546

java/google/registry/flows/host/HostCreateFlow.java

@@ -38,7 +38,7 @@ import google.registry.flows.FlowModule.TargetId;
 import google.registry.flows.TransactionalFlow;
 import google.registry.flows.annotations.ReportingSpec;
 import google.registry.model.ImmutableObject;
-import google.registry.model.domain.DomainResource;
+import google.registry.model.domain.DomainBase;
 import google.registry.model.domain.metadata.MetadataExtension;
 import google.registry.model.eppinput.ResourceCommand;
 import google.registry.model.eppoutput.CreateData.HostCreateData;
@@ -106,7 +106,7 @@ public final class HostCreateFlow implements TransactionalFlow {
     // The superordinate domain of the host object if creating an in-bailiwick host, or null if
     // creating an external host. This is looked up before we actually create the Host object so
     // we can detect error conditions earlier.
-    Optional<DomainResource> superordinateDomain =
+    Optional<DomainBase> superordinateDomain =
         lookupSuperordinateDomain(validateHostName(targetId), now);
     verifySuperordinateDomainNotInPendingDelete(superordinateDomain.orElse(null));
     verifySuperordinateDomainOwnership(clientId, superordinateDomain.orElse(null));