Skip dense_vector field usage test before 8.1

Fixes #87971.

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/50_dense_vector_field_usage.yml

@@ -49,6 +49,9 @@ setup:
 
 ---
 "Field usage":
+  - skip:
+      version: ' - 8.0.99'
+      reason: 'dense_vector field usage was added in 8.1'
   - do:
       knn_search:
         index: futest