[data.search] Remove deprecated courier:batchSearches advanced setting

[lukasolson]

Summary

Resolves #55140.

Removes the courier:batchSearches advanced setting and its usage, as it has been deprecated since 7.6 (#54539).

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
See more potential risk examples

For maintainers

• This was checked for breaking API changes and was labeled appropriately

Release note

Removes the deprecated courier:batchSearches advanced setting which allowed using the Elasticsearch _searchAPI instead of _async_search.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[mshustov]

Ok for the core changes

[Dosant]

Do we leave it as 8.0 change or do we also back port this to 7.x?

[lukasolson]

Do we leave it as 8.0 change or do we also back port this to 7.x?

Thanks for catching this, I added the 7.x and auto-backport labels out of habit. I've removed them now.

[Dosant]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 3d725a3
• Storybooks Preview
• Documentation Changes

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
data	549	547	-2

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
data	790.1KB	788.9KB	-1.2KB

Unknown metric groups

API count

id	before	after	diff
data	3502	3498	-4

API count missing comments

id	before	after	diff
data	2985	2981	-4

History

• 💔 Build #147433 failed e160be3
• 💔 Build #147157 failed 35096bf
• 💔 Build #147147 failed da702fe

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @lukasolson

[Dosant]

LGTM and ci is happy. didn't test

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create backports run node scripts/backport --pr 109350 or prevent reminders by adding the backport:skip label.

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create backports run node scripts/backport --pr 109350 or prevent reminders by adding the backport:skip label.