Comment 0 for bug 2061121

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 pressed 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.