treewide: nixfmt-rfc-style -> nixfmt

Except:
- Instances in documentation, because people in older versions
  can't switch to nixfmt yet due to it having pointed to nixfmt-classic
  before
- In code that runs based on a CI Nixpkgs version, which is also a bit
  older still
- In update script shebangs, because many of them don't pin Nixpkgs, and run
  with whatever is in NIX_PATH (and it's not easy to fix this, see
  https://github.com/NixOS/nixpkgs/issues/425551)

maintainers/scripts/auto-rebase/test/default.nix

@@ -9,7 +9,7 @@ let
     stdenvNoCC
     gitMinimal
     treefmt
-    nixfmt-rfc-style
+    nixfmt
     ;
 in
 
@@ -27,7 +27,7 @@ stdenvNoCC.mkDerivation {
   nativeBuildInputs = [
     gitMinimal
     treefmt
-    nixfmt-rfc-style
+    nixfmt
   ];
   patchPhase = ''
     patchShebangs .