Aggs: Comment on deprecated execution mode (elastic#122046) (elastic#122077)

This adds a comment to `execution_mode: global_ordinals_hash` explaining
that we're not going to remove it so we don't break anyone.

Author: nik9000

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/SignificantTermsAggregatorFactory.java

@@ -422,6 +422,10 @@ public static ExecutionMode fromString(String value, final DeprecationLogger dep
             if ("global_ordinals".equals(value)) {
                 return GLOBAL_ORDINALS;
             } else if ("global_ordinals_hash".equals(value)) {
+                /*
+                 * We have no plans to remove this so we don't break anyone, no matter
+                 * how few people still use this or how long it's been deprecated.
+                 */
                 deprecationLogger.warn(
                     DeprecationCategory.AGGREGATIONS,
                     "global_ordinals_hash",