Prefer SHA-256 ciphersuites if using old style PSKs

If we have no certificate and we are using "old style" PSKs then we will
always default to using SHA-256 for that PSK. However we may have selected
a ciphersuite that is not based on SHA-256. Therefore if we see that there
are no certificates and we have been configured for "old style" PSKs then
we should prefer SHA-256 based ciphersuites during the selection process.

Fixes #6197

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6215)