qemumonitorjsontest: Mark recently deprecated migration command in our tests

qemumonitorjsontest: Mark recently deprecated migration command in our tests

"migrate_set_downtime", "migrate_set_speed", and
"query-migrate-cache-size" were marked as deprecated in the QMP schema
in qemu 5.0. Since libvirt still actively uses them we must not mark
them as okay to be missing, but still mark them as deprecated, so that
we can add tests for deprecated commands.

The replacement of the command usage in libvirt is tracked by:
https://bugzilla.redhat.com/show_bug.cgi?id=1829543
https://bugzilla.redhat.com/show_bug.cgi?id=1829544
https://bugzilla.redhat.com/show_bug.cgi?id=1829545

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>