Instead of failing when no since, warn and use 30 days ago

More work towards solving #18 (closed)

Jobs are failing when the cache isn't working as expected, which is annoying. Instead of failing with a snarky remark, just use a sensible default. :P

https://gitlab.com/gitlab-com/gl-security/appsec/tooling/public-mr-confidential-issue-detector/-/jobs/5451358028

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Caching 2023-11-01 04:22:52 UTC for gitlab-org/gitlab-foss
FATAL: --since must be provided if the project is not already cached. Scanning all MRs ever created is not wise.
Cleaning up project directory and file based variables
00:01
ERROR: Job failed: exit code 1