Upgrading from 11.5 to 11.6 will result in 500s until unicorn workers are killed

After an apt-get install gitlab-ee when upgrading from 11.5->11.6 users will immediately start seeing 500 errors.

This is due to the ruby upgrade where the application will continue to use the old ruby which will no longer have gems on disk.

A reconfigure will issue a hup but it is not sufficient, the options to recover after the apt-get install is to either

1. Issue a hup using the hup wrapper gitlab-ctl hup unicorn
2. Restart unicorn gitlab-ctl restart unicorn

• Should it be added to our 11 changes doc? https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/update/gitlab_11_changes.md
• Does it need to go in any other changelog?
• Should we signal boost this in the release post?