CI: Move from CentOS Stream 8 to 9

CentOS Strem 8 reached an end of life, YUM repositories linked from
container images are unavailable:

    Static Analysis
    [...]
    STEP 1/4: FROM quay.io/centos/centos:stream8
    [...]
    Errors during downloading metadata for repository 'appstream':
      - Curl error (6): Couldn't resolve host name for http://mirrorlist.centos.org/?release=8-stream&arch=x86_64&repo=AppStream&infra=container [Could not resolve host: mirrorlist.centos.org]

.ci/ci-coverity.sh

@@ -9,7 +9,7 @@ set -e
 set -x
 
 os=centos
-release=stream8
+release=stream9
 repository=quay.io/centos
 
 # Override the standard tests with the Coverity scan