[Chore] Update GHC versions tested by CI

Problem: we've recently supported ghc-9.10.1, but it's not tested
by CI. Also we're still testing ghc-8.8.4, which is rather old and
causes some troubles when updating Nix dependencies.

Solution: add 9.10.1 to tested GHC versions, remove 8.8.4.

Author: gromakovsky

flake.nix

@@ -38,7 +38,7 @@
 
         hs-package-name = "with-utf8";
 
-        ghc-versions = [ "884" "8107" "902" "928" "948" "963" "981" ];
+        ghc-versions = [ "8107" "902" "928" "948" "963" "981" "9101" ];
 
         # invoke haskell.nix for each ghc version listed in ghc-versions
         pkgs-per-ghc = lib.genAttrs (map (v: "ghc${v}") ghc-versions)