treewide: use real mplus license

There has not been a lot of packages using this license, but the count
is increasing. Up until now, these packages have been marked as mit due
to how similar the two licenses are.

pkgs/data/fonts/marathi-cursive/default.nix

···
     homepage = "https://github.com/MihailJP/MarathiCursive";
     description = "Modi script font with Graphite and OpenType support";
     maintainers = with maintainers; [ mathnerd314 ];
-    license = licenses.mit; # It's the M+ license, M+ is MIT(-ish)
+    license = licenses.mplus;
     platforms = platforms.all;
   };
 }

pkgs/data/fonts/mplus-outline-fonts/default.nix

···
       homepage = "https://mplus-fonts.osdn.jp";
       maintainers = with maintainers; [ uakci ];
       platforms = platforms.all;
-      license = licenses.mit;
+      license = licenses.mplus;
     };
   };