Change required version for Get Settings transport API changes to 6.4.0

Get Settings API changes have now been backported to version 6.4, and
therefore the latest version must send and expect the extra fields when
communicating with 6.4+ code.

Relates elastic#29229 elastic#30494

Author: Thomas Callahan

server/src/main/java/org/elasticsearch/action/admin/indices/settings/get/GetSettingsRequest.java

@@ -71,7 +71,7 @@ public GetSettingsRequest(StreamInput in) throws IOException {
         indicesOptions = IndicesOptions.readIndicesOptions(in);
         names = in.readStringArray();
         humanReadable = in.readBoolean();
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_4_0)) {
             includeDefaults = in.readBoolean();
         }
     }
@@ -83,7 +83,7 @@ public void writeTo(StreamOutput out) throws IOException {
         indicesOptions.writeIndicesOptions(out);
         out.writeStringArray(names);
         out.writeBoolean(humanReadable);
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_4_0)) {
             out.writeBoolean(includeDefaults);
         }
     }

server/src/main/java/org/elasticsearch/action/admin/indices/settings/get/GetSettingsResponse.java

@@ -114,7 +114,7 @@ public void readFrom(StreamInput in) throws IOException {
         }
         ImmutableOpenMap.Builder<String, Settings> defaultSettingsBuilder = ImmutableOpenMap.builder();
 
-        if (in.getVersion().onOrAfter(org.elasticsearch.Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(org.elasticsearch.Version.V_6_4_0)) {
             int defaultSettingsSize = in.readVInt();
             for (int i = 0; i < defaultSettingsSize ; i++) {
                 defaultSettingsBuilder.put(in.readString(), Settings.readSettingsFromStream(in));
@@ -132,7 +132,7 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeString(cursor.key);
             Settings.writeSettingsToStream(cursor.value, out);
         }
-        if (out.getVersion().onOrAfter(org.elasticsearch.Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(org.elasticsearch.Version.V_6_4_0)) {
             out.writeVInt(indexToDefaultSettings.size());
             for (ObjectObjectCursor<String, Settings> cursor : indexToDefaultSettings) {
                 out.writeString(cursor.key);