Read m_m_n from cluster states from 6.7

This completes the BWC serialisation changes required for a 6.7 master to
inform other nodes of the node-level value of the `minimum_master_nodes`
setting.

Relates #37701, #37811

Author: DaveCTurner

server/src/main/java/org/elasticsearch/cluster/ClusterState.java

@@ -804,7 +804,7 @@ public static ClusterState readFrom(StreamInput in, DiscoveryNode localNode) thr
             Custom customIndexMetaData = in.readNamedWriteable(Custom.class);
             builder.putCustom(customIndexMetaData.getWriteableName(), customIndexMetaData);
         }
-        builder.minimumMasterNodesOnPublishingMaster = in.getVersion().onOrAfter(Version.V_7_0_0) ? in.readVInt() : -1;
+        builder.minimumMasterNodesOnPublishingMaster = in.getVersion().onOrAfter(Version.V_6_7_0) ? in.readVInt() : -1;
         return builder.build();
     }
 
@@ -830,7 +830,7 @@ public void writeTo(StreamOutput out) throws IOException {
                 out.writeNamedWriteable(cursor.value);
             }
         }
-        if (out.getVersion().onOrAfter(Version.V_7_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_6_7_0)) {
             out.writeVInt(minimumMasterNodesOnPublishingMaster);
         }
     }
@@ -880,7 +880,7 @@ private static class ClusterStateDiff implements Diff<ClusterState> {
             metaData = MetaData.readDiffFrom(in);
             blocks = ClusterBlocks.readDiffFrom(in);
             customs = DiffableUtils.readImmutableOpenMapDiff(in, DiffableUtils.getStringKeySerializer(), CUSTOM_VALUE_SERIALIZER);
-            minimumMasterNodesOnPublishingMaster = in.getVersion().onOrAfter(Version.V_7_0_0) ? in.readVInt() : -1;
+            minimumMasterNodesOnPublishingMaster = in.getVersion().onOrAfter(Version.V_6_7_0) ? in.readVInt() : -1;
         }
 
         @Override
@@ -894,7 +894,7 @@ public void writeTo(StreamOutput out) throws IOException {
             metaData.writeTo(out);
             blocks.writeTo(out);
             customs.writeTo(out);
-            if (out.getVersion().onOrAfter(Version.V_7_0_0)) {
+            if (out.getVersion().onOrAfter(Version.V_6_7_0)) {
                 out.writeVInt(minimumMasterNodesOnPublishingMaster);
             }
         }