Handle properly removed target platform

Currently:

Release:
A file is generated while performing a release. It contains the target platform of the release and the required packages.
This content of this file requires a computing to define which version of nuxeo is effectively used and which packages are required.
When the target platform has been removed, it's not listed.
When downloading a release from studio, a fallback to the current default target platform is performed if the release does not exists.

Snapshots:
For each branch, a file is generated, it contains the current version of the snapshot, the target version of nuxeo and the package to install. The content of this file requires a computing before to be used in Nuxeo.

Target:
For the release, no computing should be required. The computing should be performed during the release. When downloading a release's mp from studio, no fallback should be performed since no computing should be performed.
For the snapshot, if the target platform does not exists anymore, an automatic upgrade should be performed (back to current version, next LTS ?).

Steps:
A first script should be launched on prod data to identify which release are problematic (which version of nuxeo, which packages, ...).
Then, depending on this script, on second script should be started to migrate the content of the release.
An interruption of service is required.