doc, module: change async customization hooks to experimental

There is ongoing work to replace async customization hooks by a
synchronous variant.
As a result bugs/design flaws or the async variant (thread leak,
overhead) are not addressed.
Adapt stability index to avoid wrong assumptions they might move to
stable.

Refs: nodejs/loaders#201
Refs: nodejs/loaders#198
PR-URL: #60302
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: Flarna

doc/api/module.md

@@ -608,8 +608,7 @@ changes:
 
 <!-- type=misc -->
 
-> Stability: 1.2 - Release candidate (asynchronous version)
-> Stability: 1.1 - Active development (synchronous version)
+> Stability: 1.1 - Active development
 
 There are two types of module customization hooks that are currently supported: