Ruby and Rails compatibility is based on the Sidekiq major version; the upgrade notes for that major version document the supported versions of Ruby and Rails. Here are the upgrade notes for Sidekiq 5:
https://github.com/mperham/sidekiq/blob/main/5.0-Upgrade.md
For Sidekiq 5.x, it says this:
* Rails 3.2 is no longer supported (this implies Rails 4+)
* Ruby 2.0 and Ruby 2.1 are no longer supported. Ruby 2.2.2+ is required.
Use a Gemfile version lock to ensure the correct Sidekiq version. To ensure Sidekiq 5.x, use this:
gem "sidekiq", "<6"
Bundler will then select the latest version of Sidekiq Pro which support Sidekiq 5.x.