circleci: Switch centos image to stream9 tag

The plain "stream" image has not been updated for a year, and instead of
being the equivalent of a "latest" tag it just appears to be frozen.

Since there isn't an automatic centos-stream upgrade path, let's target
version 9 explicitly for now.

.circleci/config.yml

@@ -391,7 +391,7 @@ workflows:
           name: build_centos_stream
           prefix: quay.io/centos/
           dist: centos
-          release: stream
+          release: stream9
       - build:
           name: build_almalinux_8
           dist: almalinux