add version compatibility from 6.4.0 after backport, see #30319 (#30390)

Author: Paul Sanwald

server/src/main/java/org/elasticsearch/action/admin/indices/validate/query/QueryExplanation.java

@@ -75,7 +75,7 @@ public String getExplanation() {
 
     @Override
     public void readFrom(StreamInput in) throws IOException {
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_4_0)) {
             index = in.readOptionalString();
         } else {
             index = in.readString();
@@ -92,7 +92,7 @@ public void readFrom(StreamInput in) throws IOException {
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_4_0)) {
             out.writeOptionalString(index);
         } else {
             out.writeString(index);