No such interface “org.freedesktop.portal.Background”

Hello,

Having an issue with PikaBackup's background process. Using the flatpak. Running linux mint 21.1; but the package is from Ubuntu

xdg-desktop-portal - 1.14.4-1ubuntu2~22.04.1
xdg-desktop-portal-gtk - 1.14.0-1build1
flatpak - 1.12.7-1

Output of flatpak run -v when trying to configure scheduled backups

```
pika-backup:2): pika-backup-WARNING **: 10:21:10.646: src/ui/utils.rs:134:0: Background portal response: Zbus(MethodError(OwnedErrorName(ErrorName(Str(Owned("org.freedesktop.DBus.Error.UnknownMethod")))), Some("No such interface “org.freedesktop.portal.Background” on object at path /org/freedesktop/portal/desktop"), Msg { type: Error, sender: UniqueName(Str(Borrowed(":1.74"))), reply-serial: 5, body: Signature("s") }))

(pika-backup:2): pika-backup-WARNING **: 10:21:10.646: src/ui/utils.rs:377:0: Displaying error:
  Request to run in background failed
  The system does not support running Pika Backup in the background. Scheduled backup functionality and continuing backups in the background will not be available. This is either a malfunction, misconfi
…
issue in your distribution issue tracker.

ZBus Error: org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.Background” on object at path /org/freedesktop/portal/desktop
```

Also reported to xdg-desktop-portal github.