Remove references to CNBs being in preview (#468)

Since they've been generally available for some time now:
https://www.heroku.com/blog/heroku-fir-generally-available-new-platform-capabilities/

GUS-W-20242644.

Author: edmorley

README.md

@@ -8,7 +8,7 @@
 Build your Ruby and Rails application source code into an [OCI image](https://opencontainers.org/) with minimal configuration, no `Dockerfile` required. To learn more about what a CNB is, how they work, and how to use one to build your application, see the [Heroku CNB documentation][heroku-buildpacks].
 
 > [!IMPORTANT]
-> This is a collection of [Cloud Native Buildpacks][cnb], and is a component of the [Heroku Cloud Native Buildpacks][heroku-buildpacks] project, which is in preview. If you are instead looking for the Heroku Classic Buildpack for Ruby (for use on the Heroku platform), you may find it [here][classic-buildpack]. To read about differences between the buildpacks see [docs/upgrading.md](docs/upgrading.md).
+> This is a collection of [Cloud Native Buildpacks][cnb], and is a component of the [Heroku Cloud Native Buildpacks][heroku-buildpacks] project. If you are instead looking for the Heroku Classic Buildpack for Ruby, you may find it [here][classic-buildpack]. To read about differences between the buildpacks see [docs/upgrading.md](docs/upgrading.md).
 
 ## Usage