Remove Threat Insights error budget exception

Why is this change being made?

The groupthreat insights error budget exception is due for review on 2023-05-22.

This MR removes the exception. Part of gitlab-org/gitlab#383010 (closed).

Current status

• 99.92% (28-day). Down from from from 99.94% last week.
• 99.96% (7-day). Up from 99.89% last week.

Previous to the past 3 weeks, the error budget was stable on green. Since then, as explained in the 2023-05 error budget and infradev report, an inherited endpoint became problematic.

Justification

All the scope identified in the request to add an exception was delivered in the last quarter:

1. Deprecate Vulnerabilities::Feedback (gitlab-org&5629 - closed)
2. Extend the use of `security_findings` (gitlab-org&8341 - closed)

These have allowed the error budget to recover, and we can confirm that the related endpoints no longer show in the kibana error budget report.

With regards to the endpoint recently identified as problematic, work to improve its performance had already begun in Group/Sub-Group Level Dependency List - MVC (gitlab-org&8090 - closed) and Use database for project dependency list (gitlab-org&8293 - closed) but it's not excepted to finish until %16.2. Until then, our error budget is susceptible to regress to red, depending on the amount of traffic the endpoint receives.

As of last week, the two heavy users of the endpoint have reduced the amount of requests, which resulted in the budget bouncing back to 99.96% in the past week.

As a backup plan, we are monitoring the budget weekly and, if necessary, will prioritize work to improve performance on the endpoint. We want to avoid doing this now because it will be throw-away work once the two epics mentioned above are delivered.

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