Remove SGET support from Cosign

[phillebaba]

Describe what should be investigated or refactored

According to the warning message printed when calling SGET the feature is deprecated and will be removed in Zarf v1. If that is the case we need to verify that this is not breaking anything and add it to the v1 release plan.

Links to any relevant code

https://github.com/zarf-dev/zarf/blob/1c8e55f265ed8bb57cf1ee7008956725be42451f/src/pkg/utils/cosign.go#L42

Additional context

N/A