[ML] fixing bwc serialization for start datafeed request (#54652)

This bwc serialization change was previously overlooked.

Not strictly necessary as v8 is not BWC with v7.7 but only v7.last. But nice to have uniformity in the branches.

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/StartDatafeedAction.java

@@ -196,7 +196,7 @@ public class StartDatafeedAction extends ActionType<AcknowledgedResponse> {
             timeout = TimeValue.timeValueMillis(in.readVLong());
             jobId = in.readOptionalString();
             datafeedIndices = in.readStringList();
-            if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+            if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
                 indicesOptions = IndicesOptions.readIndicesOptions(in);
             } else {
                 indicesOptions = SearchRequest.DEFAULT_INDICES_OPTIONS;
@@ -286,7 +286,7 @@ public class StartDatafeedAction extends ActionType<AcknowledgedResponse> {
             out.writeVLong(timeout.millis());
             out.writeOptionalString(jobId);
             out.writeStringCollection(datafeedIndices);
-            if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+            if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
                 indicesOptions.writeIndicesOptions(out);
             }
         }