fix: use crypto/rand instead of math/rand for session certificate serial numbers

[ankur-anand]

Description

Replace the use of math/rand with cyrpto/rand for generating X.509 certificate serial Numbers in the session certificate.

Generating math/rand.Int63() has several potential issues, probably not directly evident due to the worker and target residing in the private subnet, but still not an ideal way to generate it.

1. math/rand has a predictable Output and not suitable for security-sensitive contexts.
2. Only produces 63 bits of randomness.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've documented the impact of any changes to security controls.
  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

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