The SHA256 is not a mandatory digest for DSA.

The #7408 implemented mandatory digest checking in TLS.
However this broke compatibility of DSS support with GnuTLS
which supports only SHA1 with DSS.

There is no reason why SHA256 would be a mandatory digest
for DSA as other digests in SHA family can be used as well.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9015)