[Bug #20562] Categorize `RUBY_FREE_AT_EXIT` warning as experimental

ruby.c

@@ -1779,7 +1779,7 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
     }
 
     if (getenv("RUBY_FREE_AT_EXIT")) {
-        rb_warn("Free at exit is experimental and may be unstable");
+        rb_category_warn(RB_WARN_CATEGORY_EXPERIMENTAL, "Free at exit is experimental and may be unstable");
         rb_free_at_exit = true;
     }