ruby-sidekiq 6 is in unstable

--- a/Gemfile
+++ b/Gemfile
@@ -190,7 +190,7 @@
 gem 'acts-as-taggable-on', '~> 7.0'
 
 # Background jobs
-gem 'sidekiq', '~> 5.2', '>= 5.2.7'
+gem 'sidekiq', '< 7.0', '>= 5.2.7'
 gem 'sidekiq-cron', '~> 1.0'
 gem 'redis-namespace', '~> 1.7'
 gem 'gitlab-sidekiq-fetcher', '~> 0.6.1', require: 'sidekiq-reliable-fetch'