skiff: use deprecate! instead of discontinued

Casks/s/skiff.rb

@@ -8,6 +8,8 @@ cask "skiff" do
   desc "End-to-end encrypted email, calendar, documents, and files support"
   homepage "https://skiff.com/"
 
+  deprecate! date: "2023-12-17", because: :discontinued
+
   auto_updates true
   depends_on macos: ">= :catalina"
 
@@ -19,7 +21,6 @@ cask "skiff" do
   ]
 
   caveats do
-    discontinued
     <<~EOS
       Newer version is only available in Mac App Store.
     EOS