OpenSSL: Use %y instead of %Y in VALID_DATETIME_FMT

OpenSSL: Use %y instead of %Y in VALID_DATETIME_FMT

Otherwise OpenSSL uses the GENERALIZEDTIME ASN.1 encoding
which results in LibreSSL-based clients rejecting
the certificate because they want the UTCTIME encoding
if the year is before 2050.

Example:

    fk@openbsd ~ $curl https://www.electrobsd.org/
    curl: (60) SSL certificate problem: format error in certificate's notBefore field
    [...]