Revert "Update flake.nix"

This reverts commit fc0f9d4.

flake.nix

@@ -99,7 +99,7 @@
                 # tools that work only with default compiler
                 fourmolu = fourmoluVersion;
                 hlint = "3.8";
-                haskell-language-server = "2.2.0.0";
+                haskell-language-server = "2.9.0.0";
               };
 
             # and from nixpkgs or other inputs