Fix flaky ES spec

What does this MR do and why?

This MR fixes flaky ES spec from #385513 (closed)

The reason is that we have 2 different Rspec traits: elastic and elastic_clean. Since ee/spec/elastic/migrate/20220825102900_backfill_label_ids_for_issues_spec.rb changes Elasticsearch mappings, we need to use elastic_clean instead. This trait recreates test indices for every example.

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Closes #385513 (closed)