Fix synonyms/put_synonym_rule body (#2407) (#2408)

synonyms is not a list, but a string in Solr format.

(cherry picked from commit 90b0967)

Co-authored-by: Quentin Pradet <[email protected]>

Author: github-actions[bot]

output/openapi/elasticsearch-serverless-openapi.json

@@ -42,6 +42,6 @@ export interface Request extends RequestBase {
    * The synonym rule information to update
    */
   body: {
-    synonyms: SynonymString[]
+    synonyms: SynonymString
   }
 }