Sidekiq monitor namespace change

What does this MR do?

Move Gitlab::SidekiqMonitor to Gitlab::SidekiqDaemon::Monitor namespace, per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32469#note_214211837

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/59754