Update migration guide for RC (#1206)
Update migration guide for RC (#1206)

* removed StoreKit 2 reference for RC build, added automatic migration section, small notes and fixes

* added line about filing bug reports for stuff we missed
RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios