[#372] Apply workaround for unavailable bitnami dependencies

[calohmn]

This resolves #372, applying the workaround mentioned in bitnami/charts#10539.

[stmaute]

@calohmn I think we would need to update the Readme and add the following line:
$ helm repo add bitnami-pre-2022 https://raw.githubusercontent.com/bitnami/charts/eb5f9a9513d987b519f0ecd732e7031241c50328/bitnami
In order to be able to download the MongoDB chart from the other repo.

[calohmn]

@stmaute I don't see why that would be needed. The bitnami charts are referenced in the requirements.yaml with the repository URL already. So, I don't need the bitnami repository in my local repositories list (helm repo list).

$ cd charts/ditto/
$ helm dependency update
Getting updates for unmanaged Helm repositories...
...Successfully got an update from the "https://raw.githubusercontent.com/bitnami/charts/eb5f9a9513d987b519f0ecd732e7031241c50328/bitnami" chart repository
Hang tight while we grab the latest from your chart repositories...
...Successfully got an update from the "jetstack" chart repository
...Successfully got an update from the "strimzi" chart repository
...Successfully got an update from the "grafana" chart repository
...Successfully got an update from the "elastic" chart repository
...Successfully got an update from the "prometheus-community" chart repository
...Successfully got an update from the "eclipse-iot" chart repository
...Successfully got an update from the "stable" chart repository
Update Complete. ⎈Happy Helming!⎈
Saving 1 charts
Downloading mongodb from repo https://raw.githubusercontent.com/bitnami/charts/eb5f9a9513d987b519f0ecd732e7031241c50328/bitnami
Deleting outdated charts

[stmaute]

@calohmn Yes you are right this would not be needed. I am using helm to rarely to keep all the necessary command in my head.

[calohmn]

As the ditto chart has already been updated in #374, this PR is only about the hawkBit chart now.