Investigate Botan 3.x

The only visible drawbacks of Botan 3.x are no backward compatibility with very old versions and the c++20 requirements... as Botan 2.x is supported only until the end of 2024 IMHO it is time to invest some time to look at it. Note Botan as a crypto backend should go with TLS Boost support to make HTTPS available (see #3552 (closed) for s/should/must).

Botan 2.x EOL date is 2024-12-31.