* Kinvolk now publishes Flatcar Linux images for ARM64
* For now, amd64 image must specify a plan while arm64 images
must NOT specify a plan due to how Kinvolk publishes.
Rel: https://github.com/flatcar/Flatcar/issues/872
* CoreOS Container Linux was deprecated in v1.18.3
* Continue transitioning docs and modules from supporting
both CoreOS and Flatcar "variants" of Container Linux to
now supporting Flatcar Linux and equivalents
Action Required: Update the Flatcar Linux modules `source`
to replace `s/container-linux/flatcar-linux`. See docs for
examples