styling: Don't switch app icons to symbolic for HC

styling: Don't switch app icons to symbolic for HC

  While it's true that white against black has the ultimate contrast, the
  decision to switch fullcolor icons to symbolic has a few negative implications:

    * Apps identities usually lean onto colors, switching to white symbolic
      suddenly makes it harder to identify an app.
    * Many 3rd party apps don't ship a symbolic icon making the experience feel
      unpolished/"under construction".
    * Behavior is inconsistent with gtk/libadwaita.

Part of the HC polish initiative -- https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5799

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6136

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2555>