assert: unlock the assert API

Change the Stability Index on `assert` from Locked to Stable.

PR-URL: nodejs#11304
Ref: nodejs#11200
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/assert.md

@@ -1,6 +1,6 @@
 # Assert
 
-> Stability: 3 - Locked
+> Stability: 2 - Stable
 
 The `assert` module provides a simple set of assertion tests that can be used to
 test invariants.