[gentls_cert] Update message digest

[gentls_cert] Update message digest

Debian Buster updated /etc/ssl/openssl.cnf to default to

MinProtocol = TLSv1.2
CipherString = DEFAULT@SECLEVEL=2

gentls_cert currently uses SHA1 as message digest. According to OpenSSL
documentation this only offers 80 bit of security. 80 bits is enough for
security level 1, but not 2.

The OpenSSL default MD nowadays is SHA256. This commit updates
gentls_cert to use it.

Issue was reported on the FS mailing list. The certificates created by
gentls_cert caused "md too weak" errors and clients were unable to
connect.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>