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>

.github/workflows/macos.yml

@@ -53,7 +53,7 @@ jobs:
 
       - name: Test
         run: |
-          ./build/macos/minetest.app/Contents/MacOS/minetest --run-unittests
+          ./build/macos/luanti.app/Contents/MacOS/luanti --run-unittests
 
       # Zipping the built .app preserves permissions on the contained files,
       #   which the GitHub artifact pipeline would otherwise strip away.
@@ -66,5 +66,5 @@ jobs:
 
       - uses: actions/upload-artifact@v4
         with:
-          name: minetest-macos
+          name: luanti-macos
           path: ./build/macos/*.zip