Doubled alerting threshold on registry 5xx errors again
This is a second iteration towards reducing alerts for registry 5xx errors.
The first iteration is here: !1193 (merged)
Really what we need is a metric for "number of projects throwing 5xx errors", but increasing the allowed number of errors will reduce the alerts without effecting our service levels.
This addresses: gitlab-com/gl-infra/infrastructure#7091 (closed)