Add temporary error budget exception for Threat Insights

Why is this change being made?

Add an exception entry for groupthreat insights to Stage Groups with different error budgets.

Proposed temporary budget: 99.5%.

Reason for exception

1. Work for improving the error budget is scoped out and fully planned, and funded. Completing the work will take more than a single release month, and while the work is being completed it is expected that the Error Budget will be regularly spent.

Process

Clear description of the problem that is the cause of the budget spend

By far, the largest contributor to the budget spend are requests to GET /api/:version/projects/:id/vulnerability_findings. This request relies on an early design decision to store vulnerability findings as CI pipeline artifacts, in format of JSON reports.

There are two relevant Epics in progress that will correct this problem:

1. Deprecate and remove Vulnerabilities::Feedback (gitlab-org&5629 - closed). The request above depends on a model that is being refactored-out. As part of the work, queries that depend on this model will be rewritten.
2. Extend the use of `security_findings` (gitlab-org&8341 - closed). The content required to fulfill the request above is ultimately still stored as a JSON report, which incurs the cost of fetching the report, and parsing it. This epic improves on this architecture by selectively ingesting parsed JSON information into the postgres DB.

Relevant resources showing that the work is scoped out

The epics above are prioritized, and work is under way.

Target date when the exception must be revisited

We expect gitlab-org&5629 (closed) to finish in %15.8, and gitlab-org&8341 (closed) in %15.7. This exception should be revisited in %15.9 (2022-02-22), at which time we expect to be able to rely on the regular budget spend to continue addressing the remaining violations.

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