Update `Elastic.Clients.Elasticsearch` to v9

[Piedone]

Is your feature request related to a problem?

The new, v9 version of Elastic.Clients.Elasticsearch that we use for Elasticsearch was released and we should update to it. This is not simple, the automated update by Renovate failed due to breaking changes. I briefly looked into this, and while a couple of fixes are straightforward, most of them aren't, nor are they documented in the official docs.

Describe the solution you'd like

1. Open a branch from https://github.com/OrchardCMS/OrchardCore/pull/17827 and merge https://github.com/OrchardCMS/OrchardCore/pull/17828. We shouldn't close these PRs because then Renovate would stop updating these packages.
2. Fix the incompatibilities.

Describe alternatives you've considered

I'm afraid there's no real alternative of updating. We can wait a bit more, though, so there might be some more docs on how to fix the breaking changes.

[github-actions[bot]]

We triaged this issue and set the milestone according to the priority we think is appropriate (see the docs on how we triage and prioritize issues).

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[Piedone]

The upgrade was reverted in https://github.com/OrchardCMS/OrchardCore/pull/17949, so reopening this now. See details there.