With #29331 we added support for the cluster health API to the
high-level REST client. The transport client does not support the level
parameter, and it always returns all the info needed for shards level
rendering. We have maintained that behaviour when adding support for
cluster health to the high-level REST client, to ease migration, but the
correct thing to do is to default the high-level REST client to
`cluster` level, which is the same default as when going through the
Elasticsearch REST layer.
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