Only turn on Sidekiq throttling if application settings exists

If the database has not been migrated properly, the initializer for Sidekiq
will fail to start and cause the whole Rails server to crash. This change
checks the existence for the column and allows Rails to start even
if the setting has not been added as a database column.

Closes #24452

app/models/application_setting.rb

@@ -203,6 +203,10 @@ class ApplicationSetting < ActiveRecord::Base
     ActiveRecord::Base.connection.column_exists?(:application_settings, :home_page_url)
   end
+  def sidekiq_throttling_column_exists?
+    ActiveRecord::Base.connection.column_exists?(:application_settings, :sidekiq_throttling_enabled)
+  end
+
   def domain_whitelist_raw
     self.domain_whitelist.join("\n") unless self.domain_whitelist.nil?
   end
@@ -256,6 +260,12 @@ class ApplicationSetting < ActiveRecord::Base
     ensure_health_check_access_token!
   end
+  def sidekiq_throttling_enabled?
+    return false unless sidekiq_throttling_column_exists?
+
+    sidekiq_throttling_enabled
+  end
+
   private
   def check_repository_storages

lib/gitlab/current_settings.rb

@@ -24,7 +24,7 @@ module Gitlab
     end
     def sidekiq_throttling_enabled?
-      current_application_settings.sidekiq_throttling_enabled
+      current_application_settings.sidekiq_throttling_enabled?
     end
     def fake_application_settings