Changing version check to 2.9 (opensearch-project#8416)

Update the version of BWC test from 3.0 to 2.9 for search pipeline statistic after the PR opensearch-project#8053 / commit opensearch-project@46c9a21 backported to 2.x branch.

Signed-off-by: Mingshi Liu <mingshl@amazon.com>

server/src/main/java/org/opensearch/action/admin/cluster/node/stats/NodeStats.java

@@ -193,7 +193,7 @@ public NodeStats(StreamInput in) throws IOException {
         } else {
             taskCancellationStats = null;
         }
-        if (in.getVersion().onOrAfter(Version.V_3_0_0)) { // TODO Update to 2_9_0 when we backport to 2.x
+        if (in.getVersion().onOrAfter(Version.V_2_9_0)) {
             searchPipelineStats = in.readOptionalWriteable(SearchPipelineStats::new);
         } else {
             searchPipelineStats = null;
@@ -427,7 +427,7 @@ public void writeTo(StreamOutput out) throws IOException {
         if (out.getVersion().onOrAfter(Version.V_2_9_0)) {
             out.writeOptionalWriteable(taskCancellationStats);
         }
-        if (out.getVersion().onOrAfter(Version.V_3_0_0)) { // TODO: Update to 2_9_0 once we backport to 2.x
+        if (out.getVersion().onOrAfter(Version.V_2_9_0)) {
             out.writeOptionalWriteable(searchPipelineStats);
         }
     }