Why is it not allowed to update required priority?

[Tengfei]

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Issue Info

Info	Value
Platform	e.g. ios
Platform Version	e.g. 15.0
SnapKit Version	e.g. 5.6

Issue Description

I try to update a required priority, but it fails, then I see code like this, why is it not allowed to update required priority?

SnapKit/Sources/Constraint.swift

Line 298 in 07c5d1f

 if (layoutConstraint.priority.rawValue < requiredPriority), (self.priority.constraintPriorityTargetValue != requiredPriority) { 

[Erik Fisher]

We are also seeing this issue.
Pre iOS 13, updating layout priority from required to a lower value would give a crash.
Read more here: https://stackoverflow.com/a/31186261
However, from iOS 13 and up it is now possible to update the priority and it would be great if SnapKit would allow this as well.