While implementing NRPPA-related F1AP messages, we remarked that
extension field "UL-AoA-ExtIEs" needs to be OPTIONAL (e.g., in NRPPA it
is optional in the OAI-compiled version), but it was not marked as such.
This later version 16.21.0 fixes this.
We don't work much on asn1c. The version we use has several bugs.
There is an active community out there that fixes bugs and improves
asn1c. Let's use their version.