ci: Disable migration compatibility tests for aarch64

ci: Disable migration compatibility tests for aarch64

Until 9.0 is out, we need to keep the aarch64 job disabled because the
tests always use the n-1 version of migration-test. That happens to be
broken for aarch64 in 8.2. Once 9.0 is out, it will become the n-1
version and it will bring the fixed tests.

We can revert this patch when 9.0 releases.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240118164951.30350-4-farosas@suse.de
[peterx: use _SKIPPED rather than _OPTIONAL]
Signed-off-by: Peter Xu <peterx@redhat.com>