Update sealed secret chart repo

[nitishkumar71]

In retrospect and due to #274 I think we do need to switch over to the new helm repo, but it should be a one line change if you can work on that today @nitishkumar71 ?

My understanding is that the charts have now moved to https://charts.helm.sh/stable as read-only.

Originally posted by @alexellis in #257 (comment)

Expected Behaviour

User should be able to install sealed secret form arkade

Current Behaviour

User should be able to install sealed secret form arkade

Possible Solution

Update sealed secret chart URL to https://charts.helm.sh/stable