rand_win.c: loosen version requirements for BCryptGenRandom

BCryptGenRandom() is available for Windows Vista and newer versions, see
https://docs.microsoft.com/en-us/windows/desktop/api/bcrypt/nf-bcrypt-bcryptgenrandom



Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8639)