contributing: Update policy repo to fallback to OSTC

Relate-to: eclipse/oniro-core/oniro#787
Relate-to: https://git.ostc-eu.org/oss-compliance/ip-policy/-/issues/18
Relate-to: eclipse/oniro-core/oniro!311
Forwarded: eclipse/oniro-core/docs!20


Signed-off-by: Philippe Coval <philippe.coval.ext@huawei.com>