🔊 Warn about deprecated responses usage

This was extracted from #93 and split into a separate PR.

This allows us to prepare dependent projects for the new behavior before
their logs are flooded with deprecation warnings.

lib/net/imap.rb

@@ -2501,7 +2501,7 @@ def responses(type = nil)
       elsif type
         raise ArgumentError, "Pass a block or use #clear_responses"
       else
-        # warn("DEPRECATED: pass a block or use #clear_responses", uplevel: 1)
+        warn("DEPRECATED: pass a block or use #clear_responses", uplevel: 1)
         @responses
       end
     end

test/net/imap/test_imap.rb

@@ -1104,10 +1104,10 @@ def test_responses
       assert_equal(1, imap.responses("RECENT", &:last))
       assert_raise(ArgumentError) do imap.responses("UIDNEXT") end
       # Deprecated style, without a block:
-      # assert_warn(/Pass a block.*or.*clear_responses/i) do
-      #   assert_equal(%i[Answered Flagged Deleted Seen Draft],
-      #                imap.responses["FLAGS"]&.last)
-      # end
+      assert_warn(/Pass a block.*or.*clear_responses/i) do
+        assert_equal(%i[Answered Flagged Deleted Seen Draft],
+                     imap.responses["FLAGS"]&.last)
+      end
     end
   end