org.kde.okular.appdata: Add developer_name

Fixes the following lint errors for Flathub:
```
{
    "errors": [
        "appstream-missing-developer-name"
    ]
}
```

See:
- flathub/org.kde.kdenlive#329
- flathub-infra/flatpak-builder-lint#274
- https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
- https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name
- https://mail.kde.org/pipermail/kde-devel/2024-February/002400.html

(cherry picked from commit 7c6b5fb)

shell/org.kde.okular.appdata.xml

@@ -3,6 +3,7 @@
   <id>org.kde.okular.desktop</id>
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-2.0+ and GFDL-1.3</project_license>
+  <developer_name translate="no">KDE</developer_name>
   <name>Okular</name>
   <name xml:lang="ar">اوكلار</name>
   <name xml:lang="az">Okular</name>