diagnostics_channel: mark TracingChannel as stable
This PR Promotes the TracingChannel API (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 real-world adoption: fastify, pino, mysql2, redis, ioredis, mongoose, graphql, h3, srvx, and the Vercel AI SDK publish TracingChannel events natively, and it's consumed by OpenTelemetry, Sentry, Datadog, and New Relic.
IMO, The last behavioral wrinkle in the base API was tracePromise stripping custom thenables (#59936), which was resolved semver-major in #61766 and landed in v26.0
I marked this as stable given the adoption and signals given that the API is stable and backward compatible.
I'm also suggesting we back port this to v26 docs