chore(errortracking): Loosen target apdex threshold, reading errors can be expectedly slow

Related https://gitlab.com/gitlab-org/opstrace/opstrace/-/issues/2551

This commit tunes ErrorTracking apdex targets to allow for slow read requests which are reasonably expected in our current architecture. Current telemetry available on this dashboard: https://dashboards.gitlab.net/d/observability-errortracking/observability3a-errortracking-api?orgId=1&viewPanel=20 and more details in the aforementioned issue.

To account for this in the short-term, we intend to reduce the SLI target threshold to 1.2s from 0.3s.