Remove CRIU support

[tomponline]

Please confirm

• [x] I have searched existing issues to check if an issue already exists for my feature request.

Is your feature request related to a problem? Please describe.

CRIU (live migration and stateful stop) doesn't work well with the majority of system containers due to incompatibilities with systemd. As such for most of LXD's workloads CRIU does not work at this time.

Describe the solution you'd like

We are planning to remove CRIU support until such time as this situation improves.

Describe alternatives you've considered

No response

Additional context

No response

Steps:

• [ ] Remove from LXD - updating error messages about live migration.
• [ ] Remove tests
• [ ] Remove from LXD snap (both CRIU part and the config option to enable it).
• [ ] Remove from docs if present