robo-3t: use deprecate! instead of discontinued

Casks/r/robo-3t.rb

@@ -8,6 +8,8 @@ cask "robo-3t" do
   desc "MongoDB management tool"
   homepage "https://robomongo.org/"
 
+  deprecate! date: "2023-12-17", because: :discontinued
+
   app "Robo 3T.app"
 
   uninstall quit: "Robo 3T"
@@ -20,8 +22,4 @@ cask "robo-3t" do
     "~/Library/Saved Application State/com.3tsoftwarelabs.robo3t.savedState",
     "~/Library/Saved Application State/Robo 3T.savedState",
   ]
-
-  caveats do
-    discontinued
-  end
 end