Alerts for the GitLab-managed app Prometheus are updated via helm.
Prior to this change, when alerts were updated, all helm values would
receive the current contents of `values.yaml`, with the `alerts` section
overridden, or falling back to the default chart values.

This meant that any time we made changes to `values.yaml` affecting
component versions (e.g. the versions of prometheus or alertmanager),
those would trigger a deployment of those component in the cluster,
potentially breaking the prometheus installation.

This was a very real problem, since we currently have hardcoded versions
of both prometheus server and alertmanager in `values.yaml`, and would
like to update the chart and remove the hardcoded versions in a single
change.

With this change, alert updates no longer take any values from
`values.yaml`; instead they only update alerts. We avoid reverting to
default chart values by using helm's `--reuse-values` flag instead of
the `--reset-values` flag.

See https://gitlab.com/gitlab-org/gitlab/issues/38294