Bump the reliable fetcher to 0.5.6

What does this MR do?

This allows configuring the BRPOP timeout

https://my.diffend.io/gems/gitlab-sidekiq-fetcher/0.5.5/0.5.6

The changes we're interested in are https://gitlab.com/gitlab-org/sidekiq-reliable-fetch/-/merge_requests/27. Hopefully, this helps buying us some time to work on more permanent solutions (discussion in gitlab-com/gl-infra/scalability#590 (closed))

Does this MR meet the acceptance criteria?

Conformity

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