Draft: Update gitlab-sidekiq-fetcher for Sidekiq 7 compatibility

What does this MR do and why?

This MR updates the gitlab-sidekiq-fetcher gem to work with Sidekiq >= 6.1 (including Sidekiq 7) instead of Sidekiq ~> 6.1.

This minimises the diff in Sidekiq 7 rollout on the GitLab Rails application.

Issue: gitlab-com/gl-infra/scalability#2564 (comment 1668214320)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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