Fix NPE in CatShardsRequestTests on version bump (opensearch-project#16590)

Signed-off-by: Daniel Widdis <widdis@gmail.com>

Author: dbwiddis

server/src/test/java/org/opensearch/action/admin/cluster/shards/CatShardsRequestTests.java

@@ -95,7 +95,7 @@ public void testSerializationWithOlderVersionsParametersNotSerialized() throws E
         catShardsRequest.setCancelAfterTimeInterval(TimeValue.timeValueMillis(randomIntBetween(1, 5)));
         catShardsRequest.setIndices(new String[2]);
 
-        Version version = VersionUtils.getPreviousVersion(Version.CURRENT);
+        Version version = VersionUtils.getPreviousVersion(Version.V_2_18_0);
         try (BytesStreamOutput out = new BytesStreamOutput()) {
             out.setVersion(version);
             catShardsRequest.writeTo(out);