Change BWC version for VerifyRepositoryResponse (elastic#30796)

The BWC version was previously at 7.0, because the 6.x backport had not
yet landed. Now that it has landed, this commit replaces the BWC compat
with the real version, 6.4.0.

Relates elastic#30762

server/src/main/java/org/elasticsearch/action/admin/cluster/repositories/verify/VerifyRepositoryResponse.java

@@ -140,7 +140,7 @@ public VerifyRepositoryResponse(ClusterName clusterName, DiscoveryNode[] nodes)
     @Override
     public void readFrom(StreamInput in) throws IOException {
         super.readFrom(in);
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_4_0)) {
             this.nodes = in.readList(NodeView::new).stream().map(n -> n.convertToDiscoveryNode()).collect(Collectors.toList());
         } else {
             clusterName = new ClusterName(in);
@@ -151,7 +151,7 @@ public void readFrom(StreamInput in) throws IOException {
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_4_0)) {
             out.writeList(getNodes());
         } else {
             clusterName.writeTo(out);