rubocops/cask: Check ON_SYSTEM_METHODS instead of STANZA_ORDER

- This is already here and used in multiple places. Continuing to use
  `STANZA_ORDER` will break this cop when we add `ON_SYSTEM_METHODS`
  to that list.

Library/Homebrew/rubocops/cask/no_overrides.rb

@@ -21,7 +21,7 @@ module RuboCop
 
           cask_stanzas.each do |stanza|
             # Skip if the stanza is itself an `on_*` block.
-            next unless RuboCop::Cask::Constants::STANZA_ORDER.include?(stanza.stanza_name)
+            next if ON_SYSTEM_METHODS.include?(stanza.stanza_name)
             # Skip if the stanza we detect is already in an `on_*` block.
             next if stanza.parent_node.block_type? && ON_SYSTEM_METHODS.include?(stanza.parent_node.method_name)
             # Skip if the stanza outside of a block is not also in an `on_*` block.