Fix Rake/Require offence in cleanup rake task

What does this MR do and why?

Contributes to: #391459 (closed)

This MR moves all requires into task definition of cleanup (lib/tasks/gitlab/cleanup.rake) rake task.

Screenshots or screen recordings

How to set up and validate locally

rake gitlab:cleanup:block_removed_ldap_users
rake gitlab:cleanup:moved
rake gitlab:cleanup:orphan_job_artifact_files
rake gitlab:cleanup:orphan_lfs_file_references
rake gitlab:cleanup:orphan_lfs_files
rake gitlab:cleanup:project_uploads
rake gitlab:cleanup:remote_upload_files
rake gitlab:cleanup:sessions:active_sessions_lookup_keys

bundle exec rubocop lib/tasks/gitlab/cleanup.rake
Inspecting 1 file
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1 file inspected, no offenses detected

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