Dont download whole GCP objects in clean up task

What does this MR do and why?

This resolves the memory problem we encountered in gitlab-com/gl-infra/production#17625 (comment 1897993299).

For GCP object storage provider, we had to fetch the object to be able to track the last generation attribute value before we delete it. This is needed by the rollback rake task if ever.

We encountered memory usage issues when executing this because the call to artifacts_directory.files.get would download the whole object including the content which ended up problematic for large artifact objects.

The fix here is to just use artifacts_directory.files.metadata, which is only available for fog-google, so that it will only download the object metadata.

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