Rename to Luanti (#15294)

The new header intentionally isn't in MTG stone design (or any other MTG-esque design), since we want to distance Luanti and MTG from each other. The font "undefined medium" (https://undefined-medium.com/) was used. 

ASCII art generated by https://patorjk.com/software/taag/#p=display&f=Graffiti&t=luanti
https://github.com/minetest/minetest/pull/11952#issuecomment-1013364703

---------

Co-authored-by: sfan5 <sfan5@live.de>

util/updatepo.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-# Update/create minetest po files
+# Update/create luanti po files
 
 # an auxiliary function to abort processing with an optional error
 # message
@@ -47,9 +47,9 @@ cd ..
 # First thing first, update the .pot template. We place it in the po/
 # directory at the top level. You a recent enough xgettext that supports
 # --package-name
-potfile=po/minetest.pot
+potfile=po/luanti.pot
 echo "updating pot"
-xgettext --package-name=minetest \
+xgettext --package-name=luanti \
 	--add-comments='~' \
 	--sort-by-file \
 	--add-location=file \
@@ -75,7 +75,7 @@ sed '/^#\. ~<number>.*relative_to/,/^#: /{ /^#: /!d; }' -i $potfile
 # Now iterate on all languages and create the po file if missing, or update it
 # if it exists already
 for lang in $langs ; do # note the missing quotes around $langs
-	pofile=po/$lang/minetest.po
+	pofile=po/$lang/luanti.po
 	if test -e $pofile; then
 		echo "[$lang]: updating strings"
 		msgmerge --update --sort-by-file $pofile $potfile