Mantic preseeding of LXD using incorrect track/channel

[Philip Roche]

Recent Mantic cloud image builds fail - see https://launchpad.net/~cloudware/+livefs/ubuntu/mantic/cpc/+build/604087 build log for an example.

```
+ UBUNTU_STORE_ARCH=amd64 SNAPPY_STORE_NO_CDN=1 snap download --cohort=MSAqIDE3MTI5MDgwNDcgNWIzNmM3MmIwYjVmNDIyMjQ5Y2NmY2JhNjA4MjAyNTliYzAyNjA4NWYzMjA1MWNlN2U3Y2Y3NjE5NzgwMmZkMQ== --channel=stable/ubuntu-23.10 lxd
Fetching snap "lxd"
store_download.go:142: no host system xdelta3 available to use deltas
error: cannot download snap "lxd": no snap revision available as specified
```

The was due to the default track now pointing to "5.21" instead of "latest". That change was made 20240411 which corresponds to when we started seeing failures.

The long term solution is to preseed from "latest/" for all snaps and no longer assume that "latest/" is the default.

[ Impact ]

All image with preseeding of the LXD snap will fail

[ Test Plan ]

 * Successfully build image with LXD preseeded with these changes
 * confirm LXD is preseeded/installed and works as expected.
 * Confirm that preseeding other snap like google-cloud-cli for example in the GCE images continues to preseed as expected.

[ Where problems could occur ]

 * If other snaps are also preseeded they might not be preseeded as expected. This is covered in the test plan.

[ Other Info ]

 * A temporary workaround has been made to point 5.21/stable/ubuntu-23.10 to the 5.19 LXD snap but this will expire in one month.

[Philip Roche (philroche)]

This does not affect Noble as we no longer preseed LXD in >=Noble

[Timo Aaltonen (tjaalton)]

Hello Philip, or anyone else affected,

Accepted livecd-rootfs into mantic-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/livecd-rootfs/23.10.59 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-mantic to verification-done-mantic. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-mantic. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

[Philip Roche (philroche)]

This issue affects Oracular/24.10 too.

[Philip Roche (philroche)]

Verification done for Mantic now.

 * Successfully built image with LXD preseeded with these changes
 * confirmed LXD is preseeded/installed and works as expected.
 * Confirmed that preseeding other snap like google-cloud-cli for example in the GCE images continues to preseed as expected.