Add indices.modify_data_stream to HLRC notRequiredApi allowlist

RestHighLevelClientTests.testApiNamingConventions asserts that every API
in the rest-api-spec is either implemented in the High-Level REST Client
or listed in an allowlist. The new indices.modify_data_stream spec is not
implemented in the HLRC (which is deprecated and no longer extended for
new APIs), so it must be listed in notRequiredApi - consistent with how
other recent server-only APIs (e.g. wlm_stats_list) are handled.

Fixes the gradle-check failure in :client:rest-high-level:test.

Author: Madhan

client/rest-high-level/src/test/java/org/opensearch/client/RestHighLevelClientTests.java

@@ -867,6 +867,7 @@ public void testApiNamingConventions() throws Exception {
             "indices.recovery",
             "indices.segments",
             "indices.stats",
+            "indices.modify_data_stream",
             "ingest.processor_grok",
             "nodes.info",
             "nodes.stats",