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

src/irc/irc_client.cpp

@@ -213,7 +213,7 @@ void IrcClient::start()
   auto options = Database::get_irc_server_options(this->bridge.get_bare_jid(),
                                                   this->get_hostname());
 # ifdef BOTAN_FOUND
-  this->credential_manager.set_trusted_fingerprint(options.col<Database::TrustedFingerprint>());
+  this->credential_manager->set_trusted_fingerprint(options.col<Database::TrustedFingerprint>());
 # endif
   if (Config::get("fixed_irc_server", "").empty() &&
       !options.col<Database::Address>().empty())