MGMT-15809: change base image to stream9

Now that D/S building system fully supports RHEL-9 base images (and
allows easy installation of nmstate and other libs that were previously
unsupported) we can move U/S to stream9 and D/S to ubi9-minimal.

It doesn't seem possible to easily use ubi9-minimal U/S for the
assisted-service, because installation of nmstate packages either
requires entitlements running on a RHEL node, or alternatively an access
to RedHat's internal network (which can be a major problem for
U/S development). For a better consistency we're sticking with the same
for the agent, installer, etc.

Dockerfile.assisted_installer_agent

@@ -12,7 +12,7 @@ COPY . .
 
 RUN TARGETPLATFORM=$TARGETPLATFORM make build
 
-FROM quay.io/centos/centos:stream8
+FROM quay.io/centos/centos:stream9
 ARG TARGETPLATFORM
 RUN if [[ "$TARGETPLATFORM" == "linux/amd64" || -z "$TARGETPLATFORM" ]] ; then  dnf install -y biosdevname dmidecode ; fi
 RUN if [[ "$TARGETPLATFORM" == "linux/arm64" || -z "$TARGETPLATFORM" ]] ; then  dnf install -y dmidecode ; fi