Botan 3 migration (+ Delete FindBotan.cmake)

[Phatcat]

Use the CMake builtin botan configuration for finding botan

Seems like the findx.cmake approach is getting deprecated. Botan got a configuration file for cmake back in 2023 (randombit/botan#3722), let's utilize that.

Works perfectly with a vcpkg setup; docker is getting the include dir wrong. This has been reported to botan and they're looking into it.

srp6 needs to have botan in target_link_libraries or it cannot utilize botan:

#20 2.797 [  8%] Building CXX object src/libs/srp6/CMakeFiles/srp6.dir/src/Generator.cpp.o
#20 2.808 In file included from /usr/src/ember/src/libs/srp6/src/Generator.cpp:9:
#20 2.808 /usr/src/ember/src/libs/srp6/include/srp6/Generator.h:11:10: fatal error: botan/bigint.h: No such file or directory
#20 2.808    11 | #include <botan/bigint.h>
#20 2.808       |          ^~~~~~~~~~~~~~~~
#20 2.809 compilation terminated.
#20 2.810 make[2]: *** [src/libs/srp6/CMakeFiles/srp6.dir/build.make:79: src/libs/srp6/CMakeFiles/srp6.dir/src/Generator.cpp.o] Error 1
#20 2.810 make[1]: *** [CMakeFiles/Makefile2:1881: src/libs/srp6/CMakeFiles/srp6.dir/all] Error 2
#20 2.810 make[1]: *** Waiting for unfinished jobs....
#20 4.905 [  9%] Linking CXX static library conpool.a
#20 4.972 [  9%] Built target conpool
#20 6.539 [  9%] Linking CXX static library ../../../lib/gtest.a
#20 6.612 [  9%] Built target gtest
#20 6.614 make: *** [Makefile:146: all] Error 2

libs/shared/utility/FileMD5.cpp calls for botan without having it linked in the shared libs cmakefile.

#18 18.95 [ 30%] Building CXX object src/libs/shared/CMakeFiles/shared.dir/shared/utility/FileMD5.cpp.o
#18 19.09 /usr/src/ember/src/libs/shared/shared/utility/FileMD5.cpp:10:10: fatal error: botan/bigint.h: No such file or directory
#18 19.09    10 | #include <botan/bigint.h>
#18 19.09       |          ^~~~~~~~~~~~~~~~
#18 19.09 compilation terminated.
#18 19.09 make[2]: *** [src/libs/shared/CMakeFiles/shared.dir/build.make:219: src/libs/shared/CMakeFiles/shared.dir/shared/utility/FileMD5.cpp.o] Error 1
#18 19.09 make[2]: *** Waiting for unfinished jobs....
#18 20.53 make[1]: *** [CMakeFiles/Makefile2:1817: src/libs/shared/CMakeFiles/shared.dir/all] Error 2
#18 20.53 make: *** [Makefile:146: all] Error 2

CURRENT STATUS: RFC

• Remove FindBotan.Cmake and use botan supplied botan-configure.cmake
• Patch docker + upstream botan-configuration issue to Botan: [Ubuntu x64] botan-config.cmake is failing on ubuntu multi architecture randombit/botan#4836
• Fix compiler issues (botan linking issues)
• Fix compiler warnings (use of now deprecated functions)

Fully building 0 warnings all tests passing: Addressing #75