Deprecate '--slow' Flag in favor of universal '--parallel' flag

[knqyf263]

Description

We added a --slow flag but have realized that offering control over the number of parallel processes is more preferable. Notably, in trivy k8s, there's already a --parallel flag that controls parallelism. Therefore, we plan to make the --parallel flag universally available across all targets and deprecate the --slow flag.

Upon reviewing GitHub usage, we found no instances of the --slow flag in use, suggesting minimal impact on users. However, to ensure clarity and a smooth transition, we will announce this change.