use EVP_MD_CTX_{new,free} instead of EVP_MD_CTX_{create, destroy}

for consistency with EVP_CIPHER_CTX_new() and EVP_CIPHER_CTX_free().

As the EVP_DIGESTINIT(3) man page says:

EVP_MD_CTX_create() and EVP_MD_CTX_destroy() were renamed to
EVP_MD_CTX_new() and EVP_MD_CTX_free() in OpenSSL 1.1.