step-issuer: make securityContext nullable

[wtcross]

Description

I am deploying step-issuer on OpenShift 4.20, which uses kubernetes 1.33. I am using version 1.9.11 of the chart, which is orchestrated by ArgoCD.

This change makes the following values nullable:

• securityContext
• podSecurityContext
• kubeRBACproxy.securityContext

This simplifies deployment to OpenShift since the default SCC is restricted-v3, which is used for any pods that don't have securityContext defined.

I'm not very experienced with Helm. Let me know if this is a reasonable suggestion. It will not break anything for existing chart users.

I didn't make this change in the other charts in the repo. I also did not bump the version. If the maintainers like this change let me know how you'd prefer versioning to be handled.

❤Thank you!