Bump AppVersion in node-problem-detector chart from v0.8.19 to v0.8.20

[cmontemuino]

NPD has released version 0.8.20: https://github.com/kubernetes/node-problem-detector/releases/tag/v0.8.20

Important changes:

• A new configuration file has been created: /config/readonly-monitor.json --> https://github.com/kubernetes/node-problem-detector/pull/955

[cmontemuino]

PR with version bump: https://github.com/deliveryhero/helm-charts/pull/621