bazaar: undeprecate

This formula is still needed for formulae that do `using: :bzr`.

Since the deprecation message won't be shown in the meantime, let's
replace it with a caveat.

Formula/bazaar.rb

@@ -16,7 +16,8 @@ class Bazaar < Formula
     sha256 cellar: :any_skip_relocation, high_sierra:    "cb1c0c8b5f19abef4043195d8cbd19f363a78581596de1ddcc763621964335b3"
   end
 
-  deprecate! date: "2021-08-19", because: "is not supported. Check out `breezy` instead"
+  # This formula is currently needed when downloading with `using: :bzr`.
+  # deprecate! date: "2021-08-19", because: "is not supported. Check out `breezy` instead"
 
   depends_on :macos # Due to Python 2
 
@@ -47,6 +48,13 @@ def install
     (bin/"bzr").write_env_script(libexec/"bzr", BZR_PLUGIN_PATH: "+user:#{HOMEBREW_PREFIX}/share/bazaar/plugins")
   end
 
+  def caveats
+    <<~EOS
+      This software is no longer maintained. Try `breezy` instead:
+        brew install breezy
+    EOS
+  end
+
   test do
     bzr = "#{bin}/bzr"
     whoami = "Homebrew"