CI: update Rocky Linux to 8.8

- UEFI now has to be enabled
- The root device is now `/dev/mapper/rocky-root`

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/ci.yml

@@ -531,8 +531,7 @@ jobs:
       matrix:
         box:
           - fedora/38-cloud-base
-          # v7.0.0 does not boot. v6.0.0 was not released.
-          - rockylinux/8@5.0.0
+          - rockylinux/8@8.0.0
     env:
       BOX: ${{ matrix.box }}