redmine: mark as broken

https://github.com/NixOS/nixpkgs/issues/18856

pkgs/applications/version-management/redmine/default.nix

···
     platforms = platforms.linux;
     maintainers = [ maintainers.garbas ];
     license = licenses.gpl2;
+    # Marked as broken due to needing an update for security issues.
+    # See: https://github.com/NixOS/nixpkgs/issues/18856
+    broken = true;
   };
 }