ACM requires a positive serial

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Our integration tests use this gem for generating self signed certificates, but the tests don't specify a serial on generation so they were defaulting to 0.

We started integration testing ACM intead of IAM for certs, and ACM rejects these certificates:

The serial number in the certificate is not supported by ACM.

0 is invalid per https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.2

4.1.2.2. Serial Number

The serial number MUST be a positive integer assigned by the CA to
each certificate. It MUST be unique for each certificate issued by a
given CA (i.e., the issuer name and serial number identify a unique
certificate). CAs MUST force the serialNumber to be a non-negative
integer.