Update priority and severity information

Why is this change being made?

The existing priority and severity guidance for SIRT does not align with other usages of the priority and severity labels across GitLab. The presentation is also overly complex making it difficult to evaluate an incident quickly and accurately while it is occurring. Finally, the math was incorrect in that the more areas of the CIA Triad that the incident covered the heavier it was weighted downward in severity rather than upward. Correcting the math would require an additional level of complexity as compared to giving a chart with examples and minimum priority guidance similar to what Engineering uses.

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