Upgrade from 6.2.14 -> 6.3.1 failed at katello:clean_backend_objects

Issue

Upgrading to Satellite 6.3 from 6.2 failed with the following stack trace:

out: Upgrade Step: clean_backend_objects (this may take a while) ...
 out: rake aborted!
 out: RestClient::ServiceUnavailable: Katello::Resources::Candlepin::Consumer: 503 Service Unavailable {"displayMessage":"Candlepin is in Suspend mode, please check /status resource to get more details","requestUuid":"f8d77191-65df-4519-9f15-348ad64f5203"} (GET /candlepin/consumers/?owner=XXX&include=uuid&per_page=15000&page=1)
 out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/abstract_response.rb:74:in `return!'
 out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/request.rb:495:in `process_result'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/request.rb:421:in `block in transmit'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/request.rb:413:in `transmit'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/request.rb:176:in `execute'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/request.rb:41:in `execute'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.8.0/lib/restclient/resource.rb:51:in `get'
  out: /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.4.5.62/app/lib/katello/http_resource.rb:84:in `get'
...