Changed modifier for ContentPickerValueConverter

[simonech]

Changed access modifier of ContentPickerValueConverter from internal to public.
This Value Converter was the only one defined as internal while all others are public.
It was confirmed as bug
Fixes issue #13858

[github-actions]

Hi there @simonech, thank you for this contribution! 👍

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[Zeegaan]

Looks great, thanks for the quick PR 😁

[nul800sebastiaan]

Cherry picked for 10.5 in c684b38