Fix stats test that checks for formatter tostring (#114215)

Locale may or may not be there depending on version

Author: thecoop

modules/aggregations/build.gradle

@@ -48,5 +48,4 @@ dependencies {
 
 tasks.named("yamlRestCompatTestTransform").configure({ task ->
   task.skipTest("aggregations/date_agg_per_day_of_week/Date aggregartion per day of week", "week-date behaviour has changed")
-  task.skipTest("aggregations/stats_metric_fail_formatting/fail formatting", "locale has changed")
 })

modules/aggregations/src/yamlRestTest/resources/rest-api-spec/test/aggregations/stats_metric_fail_formatting.yml

@@ -30,7 +30,7 @@ setup:
       cluster_features: "gte_v8.15.0"
       reason: fixed in 8.15.0
   - do:
-      catch: /Cannot format stat \[sum\] with format \[DocValueFormat.DateTime\(format\[date_hour_minute_second_millis\] locale\[en\], Z, MILLISECONDS\)\]/
+      catch: /Cannot format stat \[sum\] with format \[DocValueFormat.DateTime\(format\[date_hour_minute_second_millis\] locale\[(en)?\], Z, MILLISECONDS\)\]/
       search:
         index: test_date
         body: