feat: Add support fo legacy dockercfg in scan jobs

[alex123012]

Description

Now pods imagePullSecrets with secret type kubernetes.io/dockercfg are ignored when generating imagePullSecrets for scan jobs. This PR adds functionality to treat legacy kubernetes.io/dockercfg secrets and converts them to newer version (kubernetes.io/dockerconfigjson) for operator scan jobs.

I think it's not too bad to support the old types of secrets since both types (kubernetes.io/dockerconfigjson and kubernetes.io/dockercfg) perform the same function and are still supported by kubernetes.
If you don't think so - could you please explain why we can't support both types of secrets in the operator?

Checklist

• I've read the guidelines for contributing to this repository.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[chen-keinan]

@alex123012 I'm cutting a patch release and I'll review your PR afterwards

[chen-keinan]

@alex123012 thank you for the contribution lgtm 🚀 , holding merge until trivy-operator v0.13.2 officially released