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

Author: Flarna

doc/api/module.md

@@ -585,6 +585,9 @@ added: v22.8.0
 <!-- YAML
 added: v8.8.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/60302
+    description: Change stability index for async variant from Release candidate to Experimental.
   - version:
     - v23.5.0
     - v22.15.0
@@ -608,7 +611,7 @@ changes:
 
 <!-- type=misc -->
 
-> Stability: 1.2 - Release candidate (asynchronous version)
+> Stability: 1 - Experimental (asynchronous version)
 > Stability: 1.1 - Active development (synchronous version)
 
 There are two types of module customization hooks that are currently supported: