Enable Sidekiq JSON logging by default

What does this MR do and why?

Omnibus already enabled Sidekiq JSON logging by default. This change will ensure both Sidekiq clients and servers will log JSON consistently.

Using "text" will restore the original format. Previously "default" would mean "text", but that now means "json".

This came out of an investigation in #409089 (comment 1387506519).

Also relates to:

• gitlab-org/charts/gitlab#2897 (closed)
• #409034 (closed)

MR acceptance checklist

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