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>

misc/AppImageBuilder.yml

@@ -4,10 +4,10 @@ AppDir:
   path: AppDir
   app_info:
     id: net.minetest.minetest
-    name: Minetest
-    icon: minetest
+    name: Luanti
+    icon: luanti
     version: !ENV ${VERSION}
-    exec: usr/bin/minetest
+    exec: usr/bin/luanti
     exec_args: $@
   files:
     include: []
@@ -54,7 +54,7 @@ script: |
     cmake --install appimage-build
 
     # Is a backup icon location in case
-    mkdir -p AppDir/usr/share/minetest/misc
-    cp AppDir/usr/share/icons/hicolor/128x128/apps/minetest.png AppDir/usr/share/minetest/misc/minetest-xorg-icon-128.png
+    mkdir -p AppDir/usr/share/luanti/misc
+    cp AppDir/usr/share/icons/hicolor/128x128/apps/luanti.png AppDir/usr/share/luanti/misc/luanti-xorg-icon-128.png
     # Validation issues
     sed -i '/PrefersNonDefaultGPU/d' AppDir/usr/share/applications/net.minetest.minetest.desktop