Merge branch 'docs/pre-12-9-background-migration-commands' into 'master'

Add pre 12.9 background migration commands

See merge request gitlab-org/gitlab!25580

doc/update/README.md

@@ -115,17 +115,35 @@ following command:
 
 **For Omnibus installations**
 
+If using GitLab 12.9 and newer, run:
+
 ```shell
 sudo gitlab-rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
-**For installations from source**
+If using GitLab 12.8 and older, run the following using a Rails console:
 
+```ruby
+puts Sidekiq::Queue.new("background_migration").size
+Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
 ```
+
+**For installations from source**
+
+If using GitLab 12.9 and newer, run:
+
+```shell
 cd /home/git/gitlab
 sudo -u git -H bundle exec rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
+If using GitLab 12.8 and older, run the following using a Rails console:
+
+```ruby
+puts Sidekiq::Queue.new("background_migration").size
+Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
+```
+
 ## Upgrading to a new major version
 
 Major versions are reserved for backwards incompatible changes. We recommend that