ci: Update comment for migration-compat-aarch64

It turns out that we may not be able to enable this test even for the
upcoming v9.0.  Document what we're still missing.

Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Link: https://lore.kernel.org/r/20240207005403.242235-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
1 file changed