[Reporting] Research removing "Download CSV" from saved search panels in the dashboard, in favor of CSV reports.

[tsullivan]

Latest proposal is in the project doc: https://docs.google.com/document/d/1FDH9Au57-5vpU16DgpvNCbCqXjcbo8tpk_2TOtdb1Xo/edit#

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This pertains to #34571.

In the time since version 7.3 when the Download CSV feature was added, we have found a few ways that it is not a good fit in our offering of products.

1. It is not fitting with the future of Kibana's architecture, where heavy computation tasks will be run externally from the so-called UI server. Allowing the user to download a large generated file within a request puts an overly high load on the main process that serves requests.
2. It is not fitting in well in environments where Kibana is slow, or with low-bandwidth connections. Browser performance suffers when there are connections that need to stay open for a long time. When errors happen, we are not able to surface any helpful error message about what went wrong.

These are both things that the Reporting service was designed to overcome. We should focus on areas that are better at meeting the needs of the user. For CSV, this means deprecating the "Download CSV" feature and the API route, and replacing it with a link to generate a report in the background. This takes the user through the same export path as generating a CSV export from Discover, and thus has the benefit of simplifying code in the reporting plugin.

Related issues: #135274, #129524

cc @sixstringcode

[tsullivan]

Done, per the proposal doc.