Revert Markup::Heading type back to a struct

We cannot change the constant from a Struct to a
class because that breaks Marshal serialization.
It can only be changed after RDoc's serialization
does not use Marshal

Co-authored-by: Stan Lo <st0012@users.noreply.github.com>

Author: vinistock

lib/rdoc/markup/heading.rb

@@ -13,13 +13,10 @@ class Markup
     #   # Heading 1
     #   ## Heading 2
     #   ### Heading 3
-    class Heading < Element
-      #: String
-      attr_reader :text
-
-      #: Integer
-      attr_accessor :level
-
+    #
+    # IMPORTANT! Do not change this to a regular class until serialization is no longer using Marshal. If the constant
+    # changes type then deserializing the Marshal data for older Ruby versions will fail
+    Heading = Struct.new(:level, :text) do
       # A singleton RDoc::Markup::ToLabel formatter for headings.
       #: () -> RDoc::Markup::ToLabel
       def self.to_label
@@ -43,19 +40,6 @@ def to_html.handle_regexp_CROSSREF(target)
         end
       end
 
-      #: (Integer, String) -> void
-      def initialize(level, text)
-        super()
-
-        @level = level
-        @text = text
-      end
-
-      #: (Object) -> bool
-      def ==(other)
-        other.is_a?(Heading) && other.level == @level && other.text == @text
-      end
-
       # @override
       #: (untyped) -> void
       def accept(visitor)