[RFE] Prepare for Torcx deprecation

[pothos]

Current situation

The functionality offered by Torcx is going to be offered through systemd-sysext images (roadmap issue #443).

Impact

Users need to know how to use sysext images, migrate their setup away from Torcx (in case there are users) and can also migrate away from custom binaries under /opt/bin, if wanted.

Ideal future situation

• Provide docs for using sysext images (rework https://www.flatcar.org/docs/latest/container-runtimes/use-a-custom-docker-or-containerd-version/ and advise to create the /dev/null symlink to disable the upcoming shipped Docker and conatinerd sysext in advance)
• Add new deprecation section in the release notes from now on for all releases
• Blog post (Users should also ensure that they don't use the Torcx paths if they customized their containerd systemd unit), Twitter and Mailing list announcement

Implementation options

Additional information

[pothos]

Do we want to add deprecation notes to all release channels, not only Alpha?

Should we also draft a blog post to refer to in the release notes or on Mastadon/mailing-list posts?