Reassure the user that their data will be transferred to replacements

Nate Grahamrequested to merge
work/ngraham/reassuring-message into master

The dialog that informs users that a Flatpak app is EOL but has a replacement doesn't say what will happen to the user's data. This can make the choice to proceed with replacement feel scary.

Since the data does get transferred, mention that to reassure the user that nothing bad will happen.

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