Fix Botan 3 build (#9388)
* SymmetricCipher: Fix Botan 3 build
Botan commit 819cf8fe6278a19b8266f449228f02fc28a4f784 changed Botan::Cipher_Dir to be a scoped enumeration, so the users must be adapted.
This change causes no issues with Botan 2 because normal enumeration values can also be referred to the same way scoped enumeration values are accessed.
* Auto detect Botan3
* AsyncTask: Do not use `std::result_of`
`std::result_of` was deprecated in C++17 and then it was subsequently removed in C++20. One could use `std::invoke_result_t`, but let Qt figure out the return type instead.
* Collapse Botan2 and Botan3 find package into one
* Update COPYING
