Update reactive Elasticsearch healthcheck with client cluster API

[bclozel]

Once https://jira.spring.io/browse/DATAES-818 is fixed, we can leverage the client cluster API in the reactive healtcheck implementation. See #21042

[marvedly]

Now that that issue that you linked https://jira.spring.io/browse/DATAES-818 has been fixed, do you have any plans of dealing with this soon?

[bclozel]

@marvedly we're too late in the cycle to release this with 2.5.0, so we're scheduling this for 2.6.x. Thanks!

[snicoll]

FTR we had a PR around that we declined (for another reason) but we also discussed the fact that using the low-level component was more consistent, see #26140

[bclozel]

Since this was considered and declined in #26140, let's close this issue as I don't see anything else to be discussed.