Drush 12 compatibility

Problem/Motivation

• Drush 12 now expects all commandfiles in the <module-name>/src/Drush/<Commands|Generators> directory. The Drush subdirectory is a new requirement. (https://www.drush.org/12.x/commands/)
• <module-name>/drush.services.yml will be ignored (https://www.drush.org/12.x/dependency-injection/)

Remaining tasks

• Move Commands directory into a Drush directory
• Adapt namespace in UpgradeStatusCommands
• Inject dependencies using the create() method
• Adapt upgrade_status.command path in drush.services.yml