Bump UBI version from 7 to 8

Dockerfile.aro-multistage

@@ -14,7 +14,7 @@ RUN yum update -y
 COPY . ${GOPATH}/src/github.com/Azure/ARO-RP/
 RUN make aro && make e2e.test
 
-FROM ${REGISTRY}/ubi7/ubi-minimal
+FROM ${REGISTRY}/ubi8/ubi-minimal
 RUN microdnf update && microdnf clean all
 COPY --from=builder /go/src/github.com/Azure/ARO-RP/aro /go/src/github.com/Azure/ARO-RP/e2e.test /usr/local/bin/
 ENTRYPOINT ["aro"]

Dockerfile.fluentbit

@@ -1,5 +1,5 @@
 ARG REGISTRY
-FROM ${REGISTRY}/ubi7/ubi-minimal
+FROM ${REGISTRY}/ubi8/ubi-minimal
 ARG VERSION
 RUN echo -e '[td-agent-bit]\nname=td-agent-bit\nbaseurl=https://packages.fluentbit.io/centos/7/$basearch' >/etc/yum.repos.d/td-agent-bit.repo && \
     rpm --import https://packages.fluentbit.io/fluentbit.key && \