stop using embedded stable/redis chart

[thesuperzapper]

Currently, we use the (very, very) outdated stable/redis chart, I see two options:

1. Use the descendant of stable/redis which lives on bitnami/charts:redis
2. Create our own Redis YAML, and include it directly in the chart

I believe persistence is not important for how airflow uses redis (as a celery broker), so we probably don't need to include a PVC feature in our custom YAML.

PS: I am in favour of making our own YAML, but this might be a lot of effort for no reason