Replace CentOS test build with Rocky Linux

CentOS has moved to a rolling distribution.  Rocky Linux is more
like the old CentOS, being a community build of the current RedHat.
See https://wiki.rockylinux.org/

Author: daviesrob

.cirrus.yml

@@ -117,11 +117,11 @@ ubuntu_task:
   << : *TEST
 
 
-# CentOS
-centos_task:
-  name: centos-gcc
+# Rocky Linux
+rocky_task:
+  name: rockylinux-gcc
   container:
-    image: centos:latest
+    image: rockylinux:latest
     cpu: 2
     memory: 1G