Update "installation complete" artwork for Ubuntu 21.10.

[oSoMoN]

No description provided.

[oSoMoN]

Hint: to visualize the change easily, apply this patch and run the installer locally:

diff --git a/packages/ubuntu_desktop_installer/lib/main.dart b/packages/ubuntu_desktop_installer/lib/main.dart
index 9d8060c..beacee0 100644
--- a/packages/ubuntu_desktop_installer/lib/main.dart
+++ b/packages/ubuntu_desktop_installer/lib/main.dart
@@ -18,7 +18,7 @@ void main(List<String> args) {
   final subiquityServer = SubiquityServer();
 
   runWizardApp(
-    UbuntuDesktopInstallerApp(initialRoute: options['initial-route']),
+    UbuntuDesktopInstallerApp(initialRoute: '/installationcomplete'),
     options: options,
     subiquityClient: subiquityClient,
     subiquityServer: subiquityServer,

[jpnurmi]

LGTM 👍