Update prometheus chart to 9.3.x
Update the included prometheus chart to 9.3.x in order to drop support for APIs that have been removed in kubernetes 1.16.
Related to: #1562
This needs to be done a major release after the previous prometheus update: !1039 (merged) in order to ensure users are not upgrading from too old of version.
As a result this needs to happen on/after the 3.0.0 release of the GitLab chart.
The upstream PR that stopped using the old APIs is here: https://github.com/helm/charts/pull/17268