Desktop: Built-in plugins: Update Backup to v1.5.1

[personalizedrefrigerator]

Summary

Updates the backup plugin to v1.5.1 (diff).

Changes include:

• Fix conflict with multiple Joplin instances.
• Updated German language translation Update German translations in de_DE.json JackGruber/joplin-plugin-backup#96
• Added Croatian language translation Added Croatian translation JackGruber/joplin-plugin-backup#97
• Added Slovak language translation Translation: Added Slovak language JackGruber/joplin-plugin-backup#89
• Translate the plugin section label.

Testing plan

Linux/dev mode:

1. Start the desktop app.
2. Open the command palette and run ":createBackup".
3. Verify that the "Backup completed" dialog is shown.

Linux/release AppImage:

1. Open the secondary app instance (file > open secondary app instance).
2. Create a backup.
3. Verify that the backup is created in /home/username-here/JoplinBackup/alt1_default (does not overwrite the backup in /home/username-here/JoplinBackup/default).