Stop supporting botan < 2.0
The code includes quite a few “#if BOTAN_VERSION” checks. This makes the code harder to maintain, may cause bugs on untested versions (all the automatic tests currently only run with botan 2.1.0).
Furthermore, I don’t know any distribution packaging botan 1.11.x but not 2.x.
And if we forget the 1.11 branch, things get less confusing.