Prometheus chart readiness for Kubernetes 1.16

Problem to solve

Kubernetes 1.16 no longer will be serving extensions/* and apps/beta* API versions, all resources should stop using them.

GitLab-managed apps should continue to be installable and usable for Kubernetes 1.16 and up

As of https://github.com/helm/charts/pull/17268 (chart version 9.3.0), prometheus chart is now compatible with Kubernetes 1.16

Motivation

Users will start using Kubernetes 1.16 soon, especially when the cloud providers offer it.

GKE currently offers 1.14 so we are maybe a few months from 1.16 being available on GKE

Proposal

Bump prometheus chart from 6.7.3 to at least 9.3.0

Further details

We have one light one - https://gitlab.com/gitlab-org/gitlab/blob/master/qa%2Fqa%2Fspecs%2Ffeatures%2Fee%2Fbrowser_ui%2F8_monitor%2Fhealth%2Fcluster_health_spec.rb and another which is still in MR !20591 (merged), but could be run manually depending on how quickly this goes. Neither offers robust coverage, though, so we’d still want some manual testing to supplement.

/cc @syasonik @seanarnold @sengelhard @danielgruesso

Edited by Thong Kuah