in_elasticsearch: Fix string in http response code 400

[lecaros]

It's replying with 400 Forbidden; it should be 400 Bad Request

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[lecaros]

@cosmo0920 Just changed the commit messages. Thanks

[cosmo0920]

This is trivial fix but I added v3.1.7 mile stone for not overlooking to merge this PR.