biboumi: Update botan to version 3

The botan dependency has introduced a number of breaking changes with
version 3, a couple of which impact biboumi as well

This patch makes biboumi compatible with botan 3 and drops support for
botan 2 at the same time

As a consequence of the botan dependency update, C++ version has to be
upgraded to C++ 20

https://botan.randombit.net/handbook/migration_guide.html

docker/test/fedora/Dockerfile

@@ -5,6 +5,7 @@ FROM fedora:latest
 
 ENV LC_ALL=C.UTF-8
 
+# TODO: upgrade to botan3
 RUN dnf --refresh install -y \
 git \
 make \