timers: unlock the timers API

Change the Stability Index on timers from Locked to Stable.

Note that this is not intended to encourage changes to the timers API,
but to allow it when its useful for Node.js (as has happened in
violation of the documented stability level), and possibly to simplify
the stability levels by removing Locked altogether.

PR-URL: nodejs/node#11580
Ref: nodejs/node#11200
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/timers.md

@@ -1,6 +1,6 @@
 # Timers
 
-> Stability: 3 - Locked
+> Stability: 2 - Stable
 
 The `timer` module exposes a global API for scheduling functions to
 be called at some future period of time. Because the timer functions are