[Stack Monitoring] Fix displaying ES version for external collection

[miltonhultgren]

fixes #126741

When using external collection, we were trying to read the Elasticsearch version from elasticsearch.version.
However, according to the module docs the right field is elasticsearch.cluster.stats.state.version (which is aliased for cluster_state.version).

But, it seems the Metricbeat module does not actually populate this field, and it might be because the ES APIs aren't so clear about it.

elasticsearch.version is populated from the results of /_cluster/state which reports version as a simple integer.

The response from /_cluster/stats reports the version under both indices and nodes, both as lists.

The response from / reports it under version.number but with the build type included (8.3.0-SNAPSHOT).

The version shows up in a few places but not as clearly as the old internal collection documents.
As a side question, what is the version of a distributed system? A lot of the version fields in the new API response are arrays since it might be the state during a rollover to contain multiple versions.

In this PR, I'm resorting to using the first item in elasticsearch.cluster.stats.nodes.versions as the version to display in the overview.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[neptunian]

I'm thinking we should open an issue in Beats for the ES module and figure out why elasticsearch.cluster.stats.state.version isn't being populated, if elasticsearch.version is being populated correctly, and confirm which field we ought to be using. We'll probably need some help from the ES team.

I think we'd also need to update fetchClusterStats which is where the actual query lives for getClustersSummary.

I think this is fine for now, though... better than what we currently have.

[miltonhultgren]

@elasticmachine merge upstream

[miltonhultgren]

Opened elastic/beats#31481 to follow up on the Metricbeat module bug

[miltonhultgren]

@elasticmachine merge upstream

[kibana-ci]

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