Update Package PI for error budgets

Why is this change being made?

As part of the weekly SaaS performance indicators review, this MR updates the error budget PI for the Package group. As a result of this update, I opened an issue to investigate composer latency issues gitlab-org/gitlab#333166.

Also, I updated the health:reasons. This field is intended to measure how you are doing against your target, not the health of the instrumentation.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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