Voucher code generator uses insecure randomness to generate codes

[chgg-kboberg]

The voucher code generator implementation uses an insecure source of randomness:

https://github.com/voucherifyio/voucher-code-generator-java/blob/master/src/main/java/io/voucherify/generator/VoucherCodes.java#L7

java.util.Random uses Knuth's linear congruence pseudorandom number generator. Practical seed recovery attacks on this generator are known: https://hal.archives-ouvertes.fr/hal-02700791/document

While this attack requires a long-running Java process to both issue a large number of codes AND a user to obtain (in sequential order) a set of these codes, this is a drop-in fix that resolves this issue.

See #2