Remove the dangling=true filter

This has been removed from Docker Engine 24 and `docker image prune`
only deletes dangling images anyway.

Fixes https://github.com/basecamp/kamal/issues/410

test/commands/prune_test.rb

@@ -10,7 +10,7 @@ class CommandsPruneTest < ActiveSupport::TestCase
 
   test "dangling images" do
     assert_equal \
-      "docker image prune --force --filter label=service=app --filter dangling=true",
+      "docker image prune --force --filter label=service=app",
       new_command.dangling_images.join(" ")
   end