n-api: mark thread-safe function as stable

Fixes: https://github.com/nodejs/node/issues/24249
PR-URL: https://github.com/nodejs/node/pull/25556
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/node-api/test_threadsafe_function/binding.c

@@ -4,7 +4,6 @@
 // which, in turn, may affect the ABI stability of the project despite its use
 // of N-API.
 #include <uv.h>
-#define NAPI_EXPERIMENTAL
 #include <node_api.h>
 #include "../../js-native-api/common.h"