stb: switch license to mit or unlicense

https://github.com/nothings/stb/blob/master/LICENSE

pkgs/by-name/st/stb/package.nix

···
   meta = with lib; {
     description = "Single-file public domain libraries for C/C++";
     homepage = "https://github.com/nothings/stb";
-    license = licenses.publicDomain;
+    license = with licenses; [
+      mit
+      # OR
+      unlicense
+    ];
     platforms = platforms.all;
     maintainers = [ ];
   };