Issue title: Problems with KeyValueWhatsNewVersionStore
 Issue body: Hi Sven,

I tried the KeyValueWhatsNewVersionStore implementation as described in README.md and it crashed when using UserDefaults.standard, or fails when using NSUbiquitousKeyValueStore.default. The crash happens when you press the completion button.

For NSUSerdefaults it crashes with
`Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to insert non-property list object 1.0.0 for key de.tiigi.whatsnewkit.1.0.0'`

For iCloud it fails with
`[NSUbiquitousKeyValueStore setObject:forKey:]: Attempt to insert non-property value '1.0.0' of class '_SwiftValue'.
`

What would I need to do to fix this?

Max
 Comments: 
 Comment 0: Hey Max,

thanks for your detailed bug report 🙌

I've identified the issue and fixed it with a new release [1.1.1](https://github.com/SvenTiigi/WhatsNewKit/releases/tag/1.1.1)

The bug was triggered as `WhatsNew.Version` was directly stored to `UserDefaults` or `NSUbiquitousKeyValueStore` and since it doesn't support `NSCoding` cause `WhatsNew.Version` is designed as a struct, the app crashed.

The new release simply stores the String representation of a `WhatsNew.Version` 👌

Please let me know if this fix your issue
 Comment 1: Hi Sven,

Thanks for the quick reply! The issue has been fixed.

Max
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