citrix_workspace: remove old releases that will be EOLed during the lifetime of 20.09

See https://www.citrix.com/en-gb/support/product-lifecycle/milestones/receiver.html

The releases `19.{6,8,10}.0` will be EOLed in 2021 during the expected
lifetime of 20.09. As we shouldn't keep outdated software and
`19.12.0`/`20.04.0`/`20.06.0` is still maintained (and I didn't
encounter any problems with any of those releases), the deprecation
should be fine at the moment.