NIFI-9678 update Elasticsearch REST API processor integration-tests for Elasticsearch 8.x

[ChrisSamo632]

Description of PR

NIFI-9678 test integration of Elasticsearch REST API processors with 8.x.

Tested using the following integration tests in the nifi-elasticsearch-client-service module (see the updated README.md):

# Elasticsearch 5
mvnd -P integration-tests,elasticsearch-oss clean verify

# Elasticsearch 6
sleep 2s
mvnd -P integration-tests,elasticsearch-oss,elasticsearch-6 clean verify

# Elasticsearch 7.10.2
sleep 2s
mvnd -P integration-tests,elasticsearch-oss,elasticsearch-7 clean verify
sleep 2s
mvnd -P integration-tests,elasticsearch-default,elasticsearch-7 clean verify

# Elasticsearch 7.17.1
sleep 2s
mvnd -P integration-tests,elasticsearch-default,elasticsearch-7-no-oss clean verify

# Elasticsearch 8.0.1
sleep 2s
mvnd -P integration-tests,elasticsearch-default,elasticsearch-8 clean verify

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[gresockj]

What shall we do with this here?

[ChrisSamo632]

I'm generally not a fan of TODOs in code, but there's literally no deprecated Search API funcitons in Elasticsearch 8.x yet (it's too new for them to have deprecated anything, having just removed all previously deprecated stuff from 7.x).

I've Skipped the test for 8.x using the Assume but I think we should look to update this test to prove the WARNING funcitnoality for 8.x once Search API deprecations appear in Elasticsearch (so that would be a case of removing the Assume and implementing some sort of query here that caused a WARNING).

We could just remove this TODO and if branch, but I thought the TODO here would hopefully serve as a reminder to do this in future. I'd normally raise a Jira ticket to cover a TODO, but as there's no realistic way of currently implementing such a Jira ticket, it seemed like that wouldn't be sensible either.

So I think the options are:

1. Leave as-is and pick this up for a future version of Elasticsearch 8.x
2. Remove the TODO and hope someone picks this up in future
3. Raise a Jira ticket to pick this up in future, noting that it can't currently be implemented and someone will need to watch out for an appropriate Search API deprecation in order to achieve the test

[gresockj]

Thanks for the elaboration -- in light of that, I think we should keep the TODO.

[gresockj]

I successfully ran the integration tests -- it's good to know ES 8 will be supported out of the box. Aside from one question about a leftover "TODO", this test update looks good to me.

[gresockj]

Thanks for the clarification regarding the TODO, @ChrisSamo632 -- I'll go ahead and merge.