Fixing random sampling yaml rest tests for elastic#137290 (elastic#137301)

I failed to merge in main before merging elastic#137271 so I didn't pick up the
changes in elastic#137290. This accounts for them.

Author: masseyke

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_sample/30_with_data_streams.yml

@@ -285,4 +285,4 @@ teardown:
 
   - do:
       indices.get_all_sample_configuration: {}
-  - length: { $body: 0 }
+  - length: { configurations: 0 }

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_sample_stats/30_with_data_streams.yml

@@ -446,4 +446,4 @@ teardown:
 
   - do:
       indices.get_all_sample_configuration: {}
-  - length: { $body: 0 }
+  - length: { configurations: 0 }