Upgrade to Sidekiq v6.5.7

What does this MR do and why?

Fixes redis-rb deprecations so we can upgrade to redis-rb v4.8.0. Also upgrades gitlab-sidekiq-fetcher since there are also deprecated usage there.

• https://my.diffend.io/gems/sidekiq/6.4.2/6.5.7

  • Upgrading Sidekiq also fixes this polling bug: https://github.com/mperham/sidekiq/pull/5513

• https://my.diffend.io/gems/gitlab-sidekiq-fetcher/0.8.0/0.9.0

  • The only changes are from https://gitlab.com/gitlab-org/sidekiq-reliable-fetch/-/merge_requests/38)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.