Rename wrapGAppsHook to wrapGAppsHook3

[DanielSidhion]

Created from a comment in #302599 (comment).

There is a specific wrapGAppsHook4 for GTK 4. wrapGAppsHook brings GTK 3, so renaming it would make it easier for users to understand which version of the GTK they're getting with the hook they're using.

According to the comment in the PR above, it might be best to wait until #291339 is merged to avoid conflicts.

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[uninsane]

my understanding is that the eventual goal is to remove gtk{3,4} from wrapGAppsHook altogether. see the comments inside pkgs/build-support/setup-hooks/wrap-gapps-hook/default.nix:

  propagatedBuildInputs = [ makeWrapper ] ++ lib.optionals isGraphical [
    # TODO: remove this, packages should depend on GTK explicitly.
    gtk3
    librsvg
  ];

i'm not sure the precise path to get there has been elaborated. it could be:

1. replace wrapGAppsHook with explicit wrapGAppsHook{3,4} calls.
   the gtk4 half of that has already been done. the gtk3 part is what this issue would be about.
2. rename the existing wrapGAppsNoGuiHook -> wrapGAppsHook.
3. port wrapGAppsHook{3,4} users back to wrapGAppsHook.
4. delete the now unused wrapGAppsHook{3,4}.

[nyabinary]

Done :3