Ruby: changing deprecated rb_cData to rb_cObject.

Ruby 3.0 deprecated rb_cData with the intention to remove it in release 3.1.
This commit changes references of rb_cData to rb_cObject.  This was done so we
can support distributions that package Ruby 3.0, such as Fedora 34.

We also need to call rb_undef_alloc_func because we're no longer deriving from
rb_cData.  This prevents unnecessary allocations.

See:
https://docs.ruby-lang.org/en/3.0.0/doc/extension_rdoc.html

"It is recommended that klass derives from a special class called Data
(rb_cData) but not from Object or other ordinal classes.  If it doesn't,
you have to call rb_undef_alloc_func(klass)."

docs/changes.xml

@@ -50,6 +50,12 @@ points on a per-request basis during static file serving.
 </para>
 </change>
 
+<change type="feature">
+<para>
+compatibility with Ruby 3.0.
+</para>
+</change>
+
 <change type="bugfix">
 <para>
 a segmentation fault might have occurred in the PHP module if