Fix skip version for merging unmapped max (#91416)

The version was off by a minor commit and was causing random build
failures.

Closes #89994

Author: nik9000

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

@@ -206,6 +206,10 @@ setup:
 
 ---
 "Merging results with unmapped fields":
+  - skip:
+      version: " - 8.4.99"
+      reason: Fixed in 8.5.0
+
   - do:
      search:
        index: [date_test_1, date_test_2]