Fix BWC for discovery stats

The new discovery stats were pushed to the 6.x branch (currently
versioned at 6.1.0) but master was not updated to reflect this. This
impacts the mixed-cluster BWC tests because a 6.1.0 node will be trying
to send a 7.0.0 node the new discovery stats but the 7.0.0 did not yet
understand that it should be reading these when talking to a 6.1.0
node. This commit addresses this, and changes the skip version on the
discovery stats REST tests.

Author: jasontedor

core/src/main/java/org/elasticsearch/discovery/DiscoveryStats.java

@@ -44,7 +44,7 @@ public DiscoveryStats(PendingClusterStateStats queueStats, PublishClusterStateSt
     public DiscoveryStats(StreamInput in) throws IOException {
         queueStats = in.readOptionalWriteable(PendingClusterStateStats::new);
 
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_1_0)) {
             publishStats = in.readOptionalWriteable(PublishClusterStateStats::new);
         } else {
             publishStats = null;
@@ -55,7 +55,7 @@ public DiscoveryStats(StreamInput in) throws IOException {
     public void writeTo(StreamOutput out) throws IOException {
         out.writeOptionalWriteable(queueStats);
 
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_1_0)) {
             out.writeOptionalWriteable(publishStats);
         }
     }

rest-api-spec/src/main/resources/rest-api-spec/test/nodes.stats/30_discovery.yml

@@ -1,8 +1,8 @@
 ---
 "Discovery stats":
   - skip:
-      version:     " - 6.99.99"
-      reason:      "published_cluster_states_received is not (yet) in 6.x"
+      version:     " - 6.0.99"
+      reason:      "published_cluster_states_received arrived in 6.1.0"
   - do:
       cluster.state: {}