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/stress_mapgen.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 dir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 gameid=${gameid:-devtest}
-minetest=$dir/../bin/minetest
+executable=$dir/../bin/luanti
 testspath=$dir/../tests
 conf_server=$testspath/server.conf
 worldpath=$testspath/world
@@ -14,7 +14,7 @@ run () {
 	fi
 }
 
-[ -e "$minetest" ] || { echo "executable $minetest missing"; exit 1; }
+[ -e "$executable" ] || { echo "executable $executable missing"; exit 1; }
 
 rm -rf "$worldpath"
 mkdir -p "$worldpath/worldmods"
@@ -27,4 +27,4 @@ ln -s "$dir/helper_mod" "$worldpath/worldmods/"
 
 args=(--config "$conf_server" --world "$worldpath" --gameid $gameid)
 [ -n "$PROFILER" ] && args+=(--verbose)
-run "$minetest" --server "${args[@]}"
+run "$executable" --server "${args[@]}"