This fixes two bugs:
- The prune setting range was set after loading the current value. If users had a prune of (eg) 200, it would get limited to 99 before the range was raised. This is fixed by setting the range first.
- The prune setting was limited to <= the chainparams' "assumed blockchain size". There's no reason for this limit (the UX is the same either way), and there are use cases it breaks (eg, setting a prune size such that it begins pruning at some future point). Therefore, I raised it to the max value.
Backport of Core PR15801