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
- Rename all Find*.cmake files to uppercase, to make things more consistent,
and fix some issues with them (notably the REQUIRED flag)
- Rename SYSTEMDDAEMON to SYSTEMD and only use the libsystemd instead of
libsystemd-daemon because it's deprecated for a long time now
- Provide a WITH_* and WITHOUT_* switch for all optional dependencies
- Document things in the INSTALL file