Adjust BWC version on bad allocation request test

This commit adjusts the BWC version on the bad cluster allocation
explain request test as changing the API to respond with a bad request
status instead of an internal server error status was backported to 5.x
to be included in 5.6.0.

Relates elastic#25503

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.allocation_explain/10_basic.yml

@@ -1,6 +1,6 @@
 "bad cluster shard allocation explanation request":
   - skip:
-      version: " - 5.99.99"
+      version: " - 5.5.99"
       reason: response status on bad request was changed starting in 5.6.0
 
   - do: