Set Sidekiq default max concurrency to 20

What does this MR do?

The previous default of 50 was far too high. We don't use this on GitLab.com and it's mostly inertia that has left it at this value.

Rails MR: gitlab!104989 (merged)

Charts MR: gitlab-org/charts/gitlab!2868 (merged)

Related issues

Closes #5797 (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
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened