Automate bumping of charts/policy-reporter.tgz file

[viccuad]

Currently, we have checked the file charts/policy-reporter-.tgz into git.
This file is used by helm to generate the Chart.lock file for policy-reporter.
This is desirable, as we ensure that the file we consume once keeps being unmodified.

Still, it would be better to automatically update this when consuming bumps of policy-reporter

Acceptance criteria

Investigate if renovate bot can bump this file automatically, as it does for Chart.lock.
This is analogous to helm dependency update charts/kubewarden-controller which we currently do manually.