Add missing index query parameter to msearch (#4980) (#4984)

(cherry picked from commit c7bfb98)

# Conflicts:
#	output/schema/schema.json
#	output/schema/validation-errors.json

Author: pquentin

output/openapi/elasticsearch-openapi.json

@@ -251,6 +251,7 @@
         "Request: query parameter 'ignore_throttled' does not exist in the json spec",
         "Request: query parameter 'ignore_unavailable' does not exist in the json spec",
         "Request: query parameter 'include_named_queries_score' does not exist in the json spec",
+        "Request: query parameter 'index' does not exist in the json spec",
         "Request: query parameter 'routing' does not exist in the json spec"
       ],
       "response": []

output/openapi/elasticsearch-serverless-openapi.json

@@ -99,6 +99,10 @@ export interface Request extends RequestBase {
      * @server_default false
      */
     include_named_queries_score?: boolean
+    /**
+     * Comma-separated list of data streams, indices, and index aliases to use as default
+     */
+    index?: Indices
     /**
      * Maximum number of concurrent searches the multi search API can execute.
      */