Add botan 2 support

Botan 1.10 will be EOL'd this year. This patch is a rough port to botan 2 API. Also:

• Removes support for botan < 1.10 to simplify code (1.10 was released 7 years ago)
• Ports the cmake module to use pkgconfig, since botan-config doesn't exist anymore in v2.
• To minimize ifdef'd code, botan2 unique_ptr's are release'd
• Removes -ansi C(XX)FLAGS, botan 2 requires C++11

Differential Revision: https://phabricator.kde.org/D11997