test_runner: mark context.plan() as stable

This function does not require a CLI flag, does not emit a warning,
and is a fairly simple API that is already being used heavily in
the ecosystem. This commit marks context.plan() as stable.

doc/api/test.md

@@ -3345,10 +3345,12 @@ The name of the test.
 added:
   - v22.2.0
   - v20.15.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/55895
+    description: This function is no longer experimental.
 -->
 
-> Stability: 1 - Experimental
-
 * `count` {number} The number of assertions and subtests that are expected to run.
 
 This function is used to set the number of assertions and subtests that are expected to run