Skip testing new name if it isn't known

In #23638 we renamed `request_cache` to `request` in the
`_cache/clear` API. But it is only going to be committed back to
5.x so we can't test with the new name in a mixed version
cluster.

Author: nik9000

rest-api-spec/src/main/resources/rest-api-spec/test/indices.clear_cache/10_basic.yaml

@@ -5,6 +5,10 @@
 
 ---
 "clear_cache with request set to false":
+  - skip:
+      version: " - 5.3.99"
+      reason: this name was added in 5.4
+
   - do:
       indices.clear_cache:
         request: false
@@ -13,7 +17,7 @@
 "clear_cache with request_cache set to false":
   - skip:
       version: " - 5.3.99"
-      reason: deprecation was added in 5.4.0
+      reason: request_cache was deprecated in 5.4.0
       features: "warnings"
 
   - do: