libostree: add the info about signed deltas

Starting version v2020.7 `libostree` upstream provides deltas with
[signed metadata](https://github.com/ostreedev/ostree/pull/1985

)
allowing to validate the creator of the delta file itself, not only
the commit inside.

Add information about affecting this upstream change to Apertis.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>