Revert "Allow single function in FunctionScoreQuery.functions (#5576)" (#5589) (#5592)

This reverts commit 9bd939c.

(cherry picked from commit ea95e94)

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>

Author: github-actions[bot]

output/openapi/elasticsearch-openapi.json

@@ -115,7 +115,7 @@ export class FunctionScoreQuery extends QueryBase {
   /**
    * One or more functions that compute a new score for each document returned by the query.
    */
-  functions?: FunctionScoreContainer | FunctionScoreContainer[]
+  functions?: FunctionScoreContainer[]
   /**
    * Restricts the new score to not exceed the provided limit.
    */