[ML DataFrame] Use date histogram fixed_interval syntax

and remove test skip

Author: davidkyle

x-pack/plugin/src/test/resources/rest-api-spec/test/data_frame/preview_transforms.yml

@@ -130,21 +130,15 @@ setup:
 
 ---
 "Test preview returns bad request with invalid agg":
-  - skip:
-      reason:  date histo interval is deprecated
-      features: "warnings"
-
   - do:
-      warnings:
-        - "[interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future."
       catch: bad_request
       data_frame.preview_data_frame_transform:
         body: >
           {
             "source": { "index": "airline-data" },
             "pivot": {
               "group_by": {
-                "time": {"date_histogram": {"interval": "1h", "field": "time", "format": "yyyy-MM-DD HH"}}},
+                "time": {"date_histogram": {"fixed_interval": "1h", "field": "time", "format": "yyyy-MM-DD HH"}}},
               "aggs": {
                 "avg_response": {"avg": {"field": "responsetime"}},
                 "time.min": {"min": {"field": "time"}}
@@ -153,16 +147,14 @@ setup:
           }
 
   - do:
-      warnings:
-        - "[interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future."
       catch: /mixed object types of nested and non-nested fields \[time.min\]/
       data_frame.preview_data_frame_transform:
         body: >
           {
             "source": { "index": "airline-data" },
             "pivot": {
               "group_by": {
-                "time": {"date_histogram": {"interval": "1h", "field": "time", "format": "yyyy-MM-DD HH"}}},
+                "time": {"date_histogram": {"fixed_interval": "1h", "field": "time", "format": "yyyy-MM-DD HH"}}},
               "aggs": {
                 "avg_response": {"avg": {"field": "responsetime"}},
                 "time.min": {"min": {"field": "time"}}