Update buildah Version in Dev Documentation

The PostgreSQL Operator developer setup documentation now specifies
that buildah v1.14.9+ is required in order to build the PostgreSQL
Operator.  This is due to known issues in previous versions of buildah
(including v1.11.6) in which ARGs in Dockerfiles are not handled
properly, leading to inconsistent and invalid build results.

Issue: [ch9050]

Author: andrewlecuyer

docs/content/contributing/developer-setup.md

@@ -67,7 +67,7 @@ The setup target ensures the presence of:
 * [`dep`](https://golang.github.io/dep/) dependency manager
 * NSQ messaging binaries
 * `docker` container tool
-* `buildah` OCI image building tool
+* `buildah` OCI image building tool version 1.14.9+
 
 By default, docker is not configured to run its daemon. Refer to the [docker post-installation instructions](https://docs.docker.com/install/linux/linux-postinstall/) to configure it to run once or at system startup. This is not done automatically.