[8.x](backport #4012) [Integration testing] Remove AltErr and only expect 400 from ES bulk API

[mergify]

What is the problem this PR solves?

This PR expects only a 400 HTTP response code from the Elasticsearch bulk API in integration tests when invalid index name is specified. Concretely, it removes the AltErr field temporarily introduced in #4001 which was allowing the tests to accept an alternate error from Elasticsearch.

Related: elastic/elasticsearch#114869

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This is an automatic backport of pull request #4012 done by Mergify.

[mergify]

Cherry-pick of 1ae9ab6 has failed:

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	both modified:   internal/pkg/bulk/bulk_integration_test.go

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To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[jlind23]

buildkite test it

[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[ycombinator]

buildkite test this

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube