Tests: Make OldIndexBackwardsCompatibilityTests copy to fewer replicas.

We recently increased the size of bw indexes and backward compatibility tests
are now taking more time so it makes sense to ask them to do a bit less. This
commit changes the number of replicas we try to copy primaries to from (2 or 3)
to (1 or 2).

src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityTests.java

@@ -198,7 +198,7 @@ public class OldIndexBackwardsCompatibilityTests extends StaticIndexBackwardComp
     }
 
     void assertNewReplicasWork() throws Exception {
-        final int numReplicas = randomIntBetween(2, 3);
+        final int numReplicas = randomIntBetween(1, 2);
         logger.debug("Creating [{}] nodes for replicas", numReplicas);
         internalCluster().startNodesAsync(numReplicas, ImmutableSettings.builder()
                 .put("data.node", true)