Helm chart: Adding cleanup.enabled and cleanup.imagePullSecrets

[enrique-ayala]

Summary

This PR introduces two new settings to the k8up helm chart:

• cleanup.enabled: Allows users to enable/disable the cleanup of leftover role bindings and roles from older K8up versions.
• cleanup.imagePullSecrets: This allows the cleanup job to pull images from private repositories.

Checklist

For Helm Chart changes

• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• PR contains the label area:chart
• PR contains the chart label, e.g. chart:k8up
• Variables are documented in the values.yaml using the format required by Helm-Docs.
• Chart Version bumped if immediate release after merging is planned
• I have run make chart-docs
• Link this PR to related code release or other issues.

[TheBigLee]

@enrique-ayala
Thanks for this PR.

Please run make chart-docs and commit the change. This should fix the ChartLint check.
Furthermore ensure that the commits are all signed to ensure the DCO check passes.

Cheers Nicolas