Add Elasticsearch 8 to CI

Dmitry Gruzdrequested to merge
update-elasticsearch-version-for-ci into main

It turned out that we don't have Elasticsearch 8 pipeline for gitlab-elasticsearch-indexer. This MR fixes that by introducing test-es8 job and adapting tests to ES8.

This MR is a part of gitlab-org/search-team/team-tasks#109 (closed)

Edited by Dmitry Gruzd

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