Release heap resources on cancellation in TransportFieldCapabilitiesAction

[DaveCTurner]

In #92051 we added support for cancellation to TransportFieldCapabilitiesAction, but today on cancellation we still keep all the intermediate responses in memory until every node has responded. Those intermediate responses can sometimes be sizeable. This causes problems if one of the nodes is slow to respond (possibly for reasons unrelated to field caps) because then clients may cancel their requests and retry, repeatedly, leading to a build-up of partially-completed field caps requests which end up harming the coordinating node.

We should avoid keeping hold of this intermediate data any longer than necessary.

See e.g. #82685, #92987 for similar work in other areas.

[elasticsearchmachine]

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