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

CMakeLists.txt

@@ -30,7 +30,7 @@ find_library(LIBUBSAN NAMES ubsan libubsan.so.0)
 #
 ## Set various debug flags (instrumentation libs, coverage, …)
 #
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1y -pedantic -Wall -Wextra -Wconversion -fvisibility=hidden -fvisibility-inlines-hidden")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++20 -pedantic -Wall -Wextra -Wconversion -fvisibility=hidden -fvisibility-inlines-hidden")
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -fprofile-arcs -ftest-coverage --coverage")
 endif()