Update usability benchmarking per-workflow score grade categorization

Why is this change being made?

After completing the Plan benchmark report and receiving peer review, we determined that the per-workflow overall grade categorization should likely reflect the majority of the workflow sub-metric grade categories. For Plan, for example, a workflow scored 80, which is usually categorized as "Good", but all of the sub-metrics were within the "Fair" category. This was true for 2/3 of the workflows. So, we opted to categorize the overall workflow as "Fair" as well.

Within the report comments, it became clear that this kind of categorization rounding had been done for some prior benchmarks.

The goal is to establish a consistent process for these category modifications and calculations that we can use for future benchmarks.

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