Define Markup::Heading as a struct on rubies older than v4.0.0 (#1549)

Closes #1535

Reverts part of #1389

We cannot change the constant from a Struct to a class because that
breaks Marshal serialization when RDoc is working with a Ruby version
different than the latest one. It can only be changed after RDoc's
serialization does not use Marshal.

Trying to improve serialization is a larger effort, so let's revert to
fix the bug for now and we can take our time to plan that afterwards.

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

Author: vinistock

lib/rdoc/markup/heading.rb

@@ -2,6 +2,33 @@
 
 module RDoc
   class Markup
+    # IMPORTANT! This weird workaround is required to ensure that RDoc can correctly deserializing Marshal data from
+    # older rubies. Older rubies have `Heading` as a struct, so if we change it to a class, deserialization fails
+    if RUBY_VERSION.start_with?("4.")
+      class Heading < Element
+        #: String
+        attr_reader :text
+
+        #: Integer
+        attr_accessor :level
+
+        #: (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
+      end
+    else
+      Heading = Struct.new(:level, :text)
+    end
+
     # A heading with a level (1-6) and text
     #
     #  RDoc syntax:
@@ -13,13 +40,8 @@ class Markup
     #   # Heading 1
     #   ## Heading 2
     #   ### Heading 3
-    class Heading < Element
-      #: String
-      attr_reader :text
-
-      #: Integer
-      attr_accessor :level
-
+    #
+    class Heading
       # A singleton RDoc::Markup::ToLabel formatter for headings.
       #: () -> RDoc::Markup::ToLabel
       def self.to_label
@@ -43,19 +65,6 @@ 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)