Add structured logging to all importers

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We should use the new import logger: Gitlab::Import::Logger.build

We have some importers with such logging enabled, but we should enable this probably at import_service.rb level and pass it to each importer

Technical Note:

The labkit-ruby upgrade (part of upgrading to Ruby 3.0) injects info into logs - to track which deployment stage we execute on (main, cny)- after the event leaves gitlab-rails. Since it writes the stage log field, it overwrites any stage: value, so don't use this value in the logs. See !111590 (merged)