gcrypt: remove libgcrypt from the supported dependencies

.gitlab-ci.yml

CMakeLists.txt

ConfigureChecks.cmake

DefineOptions.cmake

INSTALL

cmake/Modules/FindGCrypt.cmake

config.h.cmake

doc/introduction.dox

doc/mainpage.dox

doc/threading.dox

include/libssh/bignum.h

include/libssh/callbacks.h

include/libssh/crypto.h

include/libssh/ecdh.h

include/libssh/keys.h

include/libssh/libgcrypt.h

include/libssh/pki.h

include/libssh/wrapper.h

src/CMakeLists.txt

src/bignum.c