Switched d-i boot screens to use the new emerald theme

[ Similar to my own mail about homeworld almost exactly 2 years ago! ]

Hey folks,

I've pushed changes into d-i just now so we'll use the lovely new
Emerald theme from Juliette for our boot splash screens, both for
isolinux and grub, and for mini.iso and debian-cd images.

@Juliette: I've tweaked the size and positioning of the "debian 12"
logo a little from what I found in your git repo. I hope you're ok
with this! :-)

Reference screenshots at

  https://www.einval.com/~steve/tmp/bookworm/2/

so you can see what I've done. The original placement for the logo
clashed with the menus and help text in the various menus. I'd love to
find the time to make the different menus etc. more consistent, but
probably not this release... :-/