diagnostics_channel: mark TracingChannel as stable

Promotes the diagnostics_channel.tracingChannel() factory and the
TracingChannel class from experimental to stable.

TracingChannel has been available since v18.19.0 / v19.9.0 with an
unchanged surface and now has broad adoption: fastify, pino, mysql2,
redis, ioredis, mongoose, graphql, h3, srvx, and the Vercel AI SDK
publish TracingChannel events natively, and it is consumed by
OpenTelemetry, Sentry, Datadog, and New Relic. The last behavioral
wart, tracePromise stripping custom thenables (#59936), was resolved
in #61766.

This mirrors the scoping in #45290: BoundedChannel, the bindStore /
runStores store helpers, and the concrete built-in channels stay
experimental.

Author: logaretm

doc/api/diagnostics_channel.md

@@ -237,9 +237,13 @@ diagnostics_channel.unsubscribe('my-channel', onMessage);
 added:
  - v19.9.0
  - v18.19.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/REPLACEME
+    description: Marked as stable.
 -->
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 * `nameOrChannels` {string|TracingChannel} Channel name or
   object containing all the [TracingChannel Channels][]
@@ -744,9 +748,13 @@ The scope must be used with the `using` syntax to ensure proper disposal.
 added:
  - v19.9.0
  - v18.19.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/REPLACEME
+    description: Marked as stable.
 -->
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 The class `TracingChannel` is a collection of [TracingChannel Channels][] which
 together express a single traceable action. It is used to formalize and