Drop `ClusterStateHealth#indices` when unnecessary

[DaveCTurner]

ClusterStateHealth#indices might be many MBs of data in a large cluster but it's only used by GET _cat/indices and GET _cluster/health?level={indices|shards} so it could reasonably be left empty in all other cases. Similarly ClusterIndexHealth#shards is only used by GET _cluster/health?level=shards and could be omitted elsewhere, giving at least a 2× saving in those cases too.

Relates #77466.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[NEUpanning]

I've opened a pull request for this issue in #100879