[Merge] ~juliank/livecd-rootfs:deb822-sources into livecd-rootfs:ubuntu/master

[Chad Smith]

Review: Approve

> This is correct. livecd-rootfs is pulled from the corresponding series for
image builds.

 Excellent thanks Steve/Julian. That satisfies my concern and assures me we won't be writing deb822 on focal unless we SRU'd this content back.

I've updated the cloud-init content to match what is currently in this merge request
 https://github.com/canonical/cloud-init/pull/4770.

Let us know if something changes and we'll update cloud-init's template too.

Content of deb822 comments look good to me.
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https://code.launchpad.net/~juliank/livecd-rootfs/+git/livecd-rootfs/+merge/458366
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