Updates to Observation management

Why is this change being made?

This MR makes some language updates to Observation Management to include the following:

• The addition of scoring criteria/definitions for scoring likelihood of Tier 3 (information system) risks. The current scoring table was specific to control observations and based on the number/historical observations identified as an output of control testing. However, given that information system risks may be identified independently of control testing, and subsequently, may not have a direct impact on control requirements, an additional layer is needed to allow team members across GitLab to utilize a consistent scoring method for Tier 3 risks that are not control observations
• Added a new header right before the CHER section to call out that CHER and System Health are considerations of Internal Audit and Security Compliance and are not required above and beyond likelihood and impact scoring for non-control impacting risks

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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