chore: deprecate alias Function.funext_iff in favor of funext_iff (…

…#998)

Batteries/Logic.lean

@@ -54,6 +54,7 @@ theorem funext₃ {β : α → Sort _} {γ : ∀ a, β a → Sort _} {δ : ∀ a
     {f g : ∀ a b c, δ a b c} (h : ∀ a b c, f a b c = g a b c) : f = g :=
   funext fun _ => funext₂ <| h _
 
+@[deprecated (since := "2024-10-17")]
 protected alias Function.funext_iff := funext_iff
 
 theorem ne_of_apply_ne {α β : Sort _} (f : α → β) {x y : α} : f x ≠ f y → x ≠ y :=