upterm: use deprecate! instead of discontinued

Casks/u/upterm.rb

@@ -7,6 +7,8 @@ cask "upterm" do
   desc "Terminal emulator for the 21st century"
   homepage "https://github.com/railsware/upterm"
 
+  deprecate! date: "2023-12-17", because: :discontinued
+
   app "Upterm.app"
 
   zap trash: [
@@ -17,8 +19,4 @@ cask "upterm" do
     "~/Library/Saved Application State/com.github.railsware.upterm.savedState",
     "~/.upterm",
   ]
-
-  caveats do
-    discontinued
-  end
 end