[RN] Map Fabric priorities to reconciler priorities correctly (#32847)

## Summary

This fixes how we map priorities between Fabric and the React
reconciler. At the moment, we're only considering default and discrete
priorities, when there's a larger range of priorities available.

In Fabric, we'll test supporting additional priorities soon. For that
test to do something useful, we need the new priorities to be mapped to
reconciler priorities correctly, which is what this change is done.

> [!IMPORTANT]
> At the moment, this is a no-op because Fabric is only reporting
default and discrete event priorities.

## How did you test this change?

Will test e2e on React Native on top of
https://github.com/facebook/react-native/pull/50627

The changes are gated in React Native, so we'll use that feature flag to
test this.