🐛💥 Remove accidental Data#attributes method

`#attributes` isn't part of the core `Data` API, and it never should've
been added as a public method to our polyfill.

This is a breaking API change, _but only for users on ruby 3.1._

For users on ruby 3.2+, this is _not_ a breaking change.  For newer ruby
versions, `Data#attributes` wasn't defined and this fixes YAML encoding.

Since this only affects ruby 3.1 and this was only recently added (in
v0.5.2), it's unlikely anyone has written code that uses this method.
If they have, they'll need to convert it to `#to_h`.

lib/net/imap/data_lite.rb

@@ -29,7 +29,7 @@ module Net
   class IMAP
     data_or_object = RUBY_VERSION >= "3.2.0" ? ::Data : Object
     class DataLite < data_or_object
-      def encode_with(coder) coder.map = attributes.transform_keys(&:to_s) end
+      def encode_with(coder) coder.map = to_h.transform_keys(&:to_s)        end
       def init_with(coder) initialize(**coder.map.transform_keys(&:to_sym)) end
     end
 
@@ -159,28 +159,27 @@ def marshal_load(attrs)   #{ivars} = #{attrs};  freeze end
 
       ##
       def members;     self.class.members                              end
-      def attributes;  Hash[members.map {|m| [m, send(m)] }]           end
-      def to_h(&block) attributes.to_h(&block)                         end
-      def hash;        [self.class, attributes].hash                   end
+      def to_h(&block) block ? __to_h__.to_h(&block) : __to_h__        end
+      def hash;        [self.class, __to_h__].hash                     end
       def ==(other)    self.class == other.class && to_h == other.to_h end
       def eql?(other)  self.class == other.class && hash == other.hash end
-      def deconstruct; attributes.values                               end
+      def deconstruct; __to_h__.values                                 end
 
       def deconstruct_keys(keys)
         raise TypeError unless keys.is_a?(Array) || keys.nil?
-        return attributes if keys&.first.nil?
-        attributes.slice(*keys)
+        return __to_h__ if keys&.first.nil?
+        __to_h__.slice(*keys)
       end
 
       def with(**kwargs)
         return self if kwargs.empty?
-        self.class.new(**attributes.merge(kwargs))
+        self.class.new(**__to_h__.merge(kwargs))
       end
 
       def inspect
         __inspect_guard__(self) do |seen|
           return "#<data #{self.class}:...>" if seen
-          attrs = attributes.map {|kv| "%s=%p" % kv }.join(", ")
+          attrs = __to_h__.map {|kv| "%s=%p" % kv }.join(", ")
           display = ["data", self.class.name, attrs].compact.join(" ")
           "#<#{display}>"
         end
@@ -190,7 +189,9 @@ def inspect
       private
 
       def initialize_copy(source) super.freeze end
-      def marshal_dump; attributes end
+      def marshal_dump; __to_h__ end
+
+      def __to_h__; Hash[members.map {|m| [m, send(m)] }] end
 
       # Yields +true+ if +obj+ has been seen already, +false+ if it hasn't.
       # Marks +obj+ as seen inside the block, so circuler references don't