artwork: Drop the version from the Ceratopsian and Homeworld boot screens

It's misleading for the d-i snapshot that will install testing/unstable to identify itself as Debian 13. Until Debian 14 artwork becomes available, delete the "13" from the Ceratopsian boot splash artwork, and move the logotype across to compensate.

Similarly, remove the "11" from the Homeworld artwork for completeness.

This is analogous to commit 074ca9ca "Drop the “12” version from the Emerald boot screen", but for forky.

Closes: #1118488