extensions-c9s: use match-base-evr: kernel for kernel-rt

The kernel-rt merge has already happened in CentOS Stream 9. So using
`match-base-evr` now works there. This means that rpm-ostree will
guarantee that the kernel-rt package matches the version of the base
kernel.

We don't build SCOS in the CoreOS pipelines today but let's exercise
this knob as preparation for when the change propagates to RHEL.

extensions-c9s.yaml

@@ -48,6 +48,7 @@ extensions:
       - kernel-rt-modules
       - kernel-rt-modules-extra
       - kernel-rt-devel
+    match-base-evr: kernel
   # https://github.com/openshift/machine-config-operator/pull/2456
   # https://github.com/openshift/enhancements/blob/master/enhancements/sandboxed-containers/sandboxed-containers-tech-preview.md
   # GRPA-3123