Default `PrimaryScrollController` on Desktop

Summary

Context

Description of change

Migration guide

Timeline

References

Summary

The PrimaryScrollController API has been updated to no longer automatically
attach to vertical ScrollViews on desktop platforms.

Context

Prior to this change, ScrollView.primary would default to true if a
ScrollView had an Axis.vertical scroll direction and a ScrollController
had not already been provided. This allowed for common UI patterns, like the
scroll-to-top function on iOS to work out of the box for Flutter apps.
On desktop however, this default would often cause the following assertion error: