Update import tests to clear Redis::SharedState

What does this MR do and why?

In !139395 (merged) we officially switched to use Gitlab::Redis::SharedState in Gitlab::Cache::Import::Caching, the SSOT for importer caching.

This change updates importer specs to clear the shared cached, using :clean_gitlab_redis_shared_state instead of :clean_gitlab_redis_cache.

The specs were passing without this change, which suggests they're mostly used defensively against cache-related bugs. Only one spec has had the metadata removed altogether, spec/lib/gitlab/github_import/representation/diff_note_spec.rb, as the test subject doesn't use caching.

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