monitoring: change relevant hard threshold alerts to ratio-based alerts

Created by: bobheadxi

https://github.com/sourcegraph/sourcegraph/issues/12158 (PR: https://github.com/sourcegraph/sourcegraph/pull/12756) changes many of sourcegraph-frontend's hard-threshold alerts to ratio-based alerts. The rationale is roughly:

In general some of the noisiest alerts in #alerts-cloud are those alerts with hard thresholds, ie "Y+ errors in X minutes" - on larger instances like Sourcegraph Cloud, this could mean we fire alerts on issues that only affect a very small number of users.

We should take a look at the remaining noisy alerts on #alerts-cloud and see which ones we can improve for large Sourcegraph deployments by converting them to ratio-based alerts.