feishin: use electron_31

As originally mentioned in NixOS#331868, feishin now [1] depends on Electron v31,
which was not packaged at the time (NixOS#325428).
It has since been fixed (by NixOS#333907).

[1]: https://github.com/jeffvli/feishin/blob/f2beeef/package.json#L234

pkgs/by-name/fe/feishin/package.nix

@@ -3,7 +3,7 @@
   stdenv,
   buildNpmPackage,
   fetchFromGitHub,
-  electron_30,
+  electron_31,
   darwin,
   copyDesktopItems,
   makeDesktopItem,
@@ -20,7 +20,7 @@ let
     hash = "sha256-UOY0wjWGK7sal/qQbbkHjFUIA49QtbO+Ei6hSTOyHWk=";
   };
 
-  electron = electron_30;
+  electron = electron_31;
 in
 buildNpmPackage {
   inherit pname version;