Change the sidekiq warning to raise an error instead

What does this MR do?

This will change a deprecation warning introduced in %14.8 to now throw an error, since the deprecated feature is removed in %15.0.

Related issues

#6785 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated: gitlab!86234 (merged)
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened: gitlab-org/charts/gitlab!2531 (merged)

Closes #6785 (closed)