[Merge] ~philroche/livecd-rootfs/+git/livecd-rootfs:bugfix/lp-2061121-snap-preseed-latest into livecd-rootfs:ubuntu/mantic

[Philip Roche]

we only encode it for LTS releases  -see https://ubuntu-archive-team.ubuntu.com/seeds/ubuntu.mantic/server 

This change does not affect the format used in LTS like `snap:lxd=5.0/stable/ubuntu-22.04`. That still matches the first case statement.
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https://code.launchpad.net/~philroche/livecd-rootfs/+git/livecd-rootfs/+merge/464199
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