Fix the max psk len for TLSv1.3

If using an old style TLSv1.2 PSK callback then the maximum possible PSK
len is PSK_MAX_PSK_LEN (256) - not 64.

Fixes #7261

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7267)