return ssh status in migrations_check

[Panaetius]

Add support in /cache.migrations_check endpoint on whether a project supports SSH or not.

The basic version of this is just seeing if the template version used is higher than x.

A nice to have would be checking if port 22 is open/exposed in a session, maybe using https://github.com/NVIDIA/container-canary . Or checking that an image is based on an appropriate base image.

[Panaetius]

This could also mean reviving SwissDataScienceCenter/amalthea#239 so you'd get an error when starting a session with SSH enabled but it doesn't support SSH

[Panaetius]

An easy solution would be to just extend the template manifest.yaml with a ssh_supported flag that defaults to false and can be set by template authors. We could also have a setting in renku.ini to force enable ssh support for a project.