DOC: update current devel deprecation regarding initramfs stuff
DOC: update current devel deprecation regarding initramfs stuff

The new solution around the target userspace container, the upgrade
initramfs, and the target initramfs has been merged in leapp-repository
upstream for el7toel8. With that, 4 models has become deprecated as
these have been replaced by new models.

Deprecated models:
  - RequiredTargetUserspacePackages
  - RequiredUpgradeInitramPackages
  - UpgradeDracutModule
  - InitrdIncludes

leapp-to/prototype,leapp-to/prototype,leapp-to/prototype,leapp-to/prototype