Adapt kubeapps chart to stable deprecation

[carrodher]

Description of the change

Bitnami charts are no longer updated in the stable repository. This PR updates some references/links in the docs and also the requirements.yaml to pull the dependencies from the updated repository.

+info: helm/charts#20969

Possible drawbacks

As the bitnami repository already contains all the bitnami contributed charts, there are no drawbacks because:

• Changes in the docs shouldn't affect anything
• Charts included as deps are already in the "new" repository (new in terms of kubeapps requirements.yaml)

[andresmgot]

LGTM

[carrodher]

Just for the records.
826b756 is needed when testing with Helm 2.14 because a bug/issue in Helm itself solved in 2.15.

I was able to reproduce it using Helm 2.14.0:

$ helm214 dep update
Error: no repository definition for https://charts.bitnami.com/bitnami, https://charts.bitnami.com/bitnami. Please add them via 'helm repo add'

while the issue doesn’t appear in Helm 2.16.1:

$ helm216 dep update
Hang tight while we grab the latest from your chart repositories...
...Successfully got an update from the "stable" chart repository
Update Complete.
Saving 2 charts
Downloading mongodb from repo https://charts.bitnami.com/bitnami
Downloading postgresql from repo https://charts.bitnami.com/bitnami
Deleting outdated charts

Solved here: helm/helm#6258

We fixed it adding the repository because it's useful for us to test kubeapps using previous Helm versions.