Allow single percent in PercentilesAggregation (#5552) (#5578)

(cherry picked from commit 8bef67e)

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

Author: github-actions[bot]

output/openapi/elasticsearch-openapi.json

@@ -227,7 +227,7 @@ export class PercentilesAggregation extends FormatMetricAggregationBase {
   /**
    * The percentiles to calculate.
    */
-  percents?: double[]
+  percents?: double | double[]
   /**
    * Uses the alternative High Dynamic Range Histogram algorithm to calculate percentiles.
    */