Set Sidekiq default max concurrency to 20

What does this MR do?

The previous default of 25 was fine, but didn't match Omnibus or source installs, which used 50. Let's consolidate at 20 and fix the documentation to have the correct value.

Rails MR: gitlab-org/gitlab!104989 (merged)

Omnibus MR: gitlab-org/omnibus-gitlab!6536 (merged)

Related issues

For gitlab-org/omnibus-gitlab#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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

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 omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.