Reword the "Prioritized Recommendations" section of the WPT coverage evaluation to tier significance

[DanielRyanSmith]

Suggestion from @vmpstr:

It would be great if the qualification of priority also indicated how important this is or how much of a testing hole this is.

It could be useful to word the "Prioritized Recommendations" section with clear importance levels for each suggested addition (e.g. "High", "Low", "Crucial", "Optional")

[DanielRyanSmith]

The changes in #5743 tangentially address this by overhauling this summary section. We could still add a priority level for all test suggestions, but it might be relatively arbitrary, and we'd need to do thorough verification that Gemini can even get the prioritization correct.