Use Sparkle sorting/filtering in #livecheck_min_os

The `#livecheck_min_os` cask audit method manually replicates some of
the `Sparkle` strategy's behavior but in an incomplete way that has
lead to inappropriate audit failures at times. This reimplements it
to use `Livecheck` methods, so it will align with the `Sparkle`
strategy's behavior.

Library/Homebrew/cask/audit.rb

@@ -603,31 +603,25 @@ module Cask
       return if cask.livecheck.strategy_block.present? &&
                 cask.livecheck.strategy_block.parameters[0] == [:opt, :items]
 
-      out, _, status = curl_output("--fail", "--silent", "--location", cask.livecheck.url)
-      return unless status.success?
-
-      require "rexml/document"
-
-      xml = begin
-        REXML::Document.new(out)
-      rescue REXML::ParseException
-        nil
-      end
-
-      return if xml.blank?
-
-      item = xml.elements["//rss//channel//item"]
-      return if item.blank?
-
-      min_os = item.elements["sparkle:minimumSystemVersion"]&.text
-      min_os = "11" if min_os == "10.16"
-      return if min_os.blank?
+      content = Homebrew::Livecheck::Strategy.page_content(cask.livecheck.url)[:content]
+      return if content.blank?
 
       begin
-        MacOSVersion.new(min_os).strip_patch
-      rescue MacOSVersion::Error
-        nil
+        items = Homebrew::Livecheck::Strategy::Sparkle.sort_items(
+          Homebrew::Livecheck::Strategy::Sparkle.filter_items(
+            Homebrew::Livecheck::Strategy::Sparkle.items_from_content(content),
+          ),
+        )
+      rescue
+        return
       end
+      return if items.blank?
+
+      min_os = items[0]&.minimum_system_version&.strip_patch
+      # Big Sur is sometimes identified as 10.16, so we override it to the
+      # expected macOS version (11).
+      min_os = MacOSVersion.new("11") if min_os == "10.16"
+      min_os
     end
 
     sig { returns(T.nilable(MacOSVersion)) }