Skip max_buckets test when it is flaky

Before elastic#57042 the max_buckets test would consistently pass because the
request would consistently fail. In particular, the request would fail on
the data node. After elastic#57042 it only fails on the coordinating node. When
the max_buckets test is run in a mixed version cluster it consistently
fails on *either* the data node or the coordinating node. Except when
the coordinating node is missing elastic#43095. In that case if the one data
node has elastic#57042 and one does not, *and* the one that doesn't gets the
request first, fails it as expected, and then the coordinating node
retries the request on the node with elastic#57042. When that happens the
request fails mysteriously with "partial shard failures" as the error
message but not partial failures reported. This is *exactly* the bug
fixed in elastic#43095.

This updates the test to be skipped in mixed version clusters without
 elastic#43095 because they *sometimes* fail the test spuriously. The request
fails in those cases, just like we expect, but with a mysterious error
message.

Closes elastic#57657

Author: nik9000

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/240_max_buckets.yml

@@ -76,8 +76,8 @@ setup:
 ---
 "Max bucket":
   - skip:
-      version: " - 6.99.99"
-      reason:  search.max_buckets limit has been added in 7.0
+      version: " - 7.2.99"
+      reason:  search.max_buckets limit has been added in 7.0 but this fails spuriously without https://github.com/elastic/elasticsearch/pull/43095 which is in 7.3.0
 
   - do:
       cluster.put_settings: