Upgrade elasticsearch ruby client and remove 7.x related logics

[mashhurs]

TODO: add changelog

• Closes: Upgrade elasticsearch client, remove 7.x patch logics #212

[alexcams]

Just a minor suggestion and a question about test coverage.

[alexcams]

LGTM! when CI is green. Maybe someone else has something to say.

[mashhurs]

/pr_prepare release: minor

• Drop a support for Logstash 7.x by requiring elasticsearch gem >= 8. Logstash 8+ continues to work as before.

[alexcams]

Hey @mashhurs!

You'd need to remove the existing changelog entry for the automation to work correctly.

The workflow detects if there have been changes in the changelog but not in version file when the PR is open and adds a comment:
This PR updates the changelog but does not update the plugin version. Please check if the current plugin version is already published on [RubyGems](https://rubygems.org/). If it is, a version bump is needed.

Since this PR was open before main branch had the workflow added, that preventing message hasn't been posted.

[mashhurs]

Hey @mashhurs!

You'd need to remove the existing changelog entry for the automation to work correctly.

The workflow detects if there have been changes in the changelog but not in version file when the PR is open and adds a comment: This PR updates the changelog but does not update the plugin version. Please check if the current plugin version is already published on [RubyGems](https://rubygems.org/). If it is, a version bump is needed.

Since this PR was open before main branch had the workflow added, that preventing message hasn't been posted.

Ah, thank you for letting know, didn't notice. I have just removed it.

[mashhurs]

CI failures are due to the elasticsearch-ruby client v9.4.1 version which is setting duplicate headers (Content-Type and content-type), details are here -
elastic/elasticsearch-ruby@ff9ec8f#r186029959

I will hold off merging till the point we will have a clear direction either sanitize duplicate headers or fix in the upstream.