feat: increase DCBACKUP_VERSION with PGP-contacts (#6929)

This way, we prevent users from running into problems when importing an
old backup.

Note that this is not strictly necessary, but a decision of assuming
that this is better for usability - importing a new backup into an old
DC version would mostly work fine, but there would be weird bugs. E.g.,
after updating the imported profile to PGP-contacts, the migration
wouldn't run a second time, so that all peerstate changes since the
device was updated would get lost.

Desktop|Android
-|-

![image](https://github.com/user-attachments/assets/b9fc6f3a-f778-4e64-ad2c-393d5c756ded)
|
![image](https://github.com/user-attachments/assets/65aceb7e-eeba-45e3-8f00-0c676a218f9d)

Not sure why Desktop shows a nice error message, while Android shows a
cryptic "IMEX failed to complete", though for this release, it's too
late to change this (except if we want to make a quick release before
PGP-contacts, which improves the error message).

_Edit:_ It's because I used Add-second-device for Desktop, and
import-file for Android. Would be nice to improve the error message when
importing a backup, but this is out of scope for this PR. I created an
issue: #6930 This is not a
blocker for the PGP-contacts release, because as I said, it's too late
to change this except if we want to make another release before the
PGP-contacts release.

Author: Hocuri

src/qr.rs

@@ -41,7 +41,7 @@ pub(crate) const DCBACKUP_SCHEME_PREFIX: &str = "DCBACKUP";
 
 /// Version written to Backups and Backup-QR-Codes.
 /// Imports will fail when they have a larger version.
-pub(crate) const DCBACKUP_VERSION: i32 = 2;
+pub(crate) const DCBACKUP_VERSION: i32 = 3;
 
 /// Scanned QR code.
 #[derive(Debug, Clone, PartialEq, Eq)]