Don't compile rb_undefine_finalizer on Ruby 3.4.0 and 3.4.1

Ruby 3.4.0 and 3.4.1 have a bug where rb_undefine_finalizer is missing.
This may fail to compile on some platforms.

optional/capi/ext/finalizer_spec.c

@@ -10,7 +10,13 @@ static VALUE define_finalizer(VALUE self, VALUE obj, VALUE finalizer) {
 }
 
 static VALUE undefine_finalizer(VALUE self, VALUE obj) {
+// Ruby 3.4.0 and 3.4.1 have a bug where rb_undefine_finalizer is missing
+// See: https://bugs.ruby-lang.org/issues/20981
+#if RUBY_API_VERSION_CODE == 30400 && (RUBY_VERSION_TEENY == 0 || RUBY_VERSION_TEENY == 1)
+    return Qnil;
+#else
   return rb_undefine_finalizer(obj);
+#endif
 }
 
 void Init_finalizer_spec(void) {