Adjust BWC test version for disk usage API

Relates #74051

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.stats/50_disk_usage.yml

@@ -1,8 +1,8 @@
 ---
 setup:
   - skip:
-      version: " - 7.99.99"
-      reason: analyze index disk usage API is introduced in 8.0
+      version: " - 7.14.99"
+      reason: analyze index disk usage API is introduced in 7.15
 
 ---
 "Disk usage stats":