Retire Desktop Bridge era content, modernize WinRT API setup guide

Rebased on main; all review comments addressed:
- Broken anchor stubs added (#net-6-and-later-use-the-target-framework-moniker-option, #conditional-compilation)
- Toast notification identity note added
- In-repo references to deleted pages updated (TOC, modernize-packaged-apps, best-practices, packaging/index)
- Merge conflict in redirection.json resolved

Co-authored-by: Copilot <[email protected]>

Author: GrantMeStrength

.openpublishing.redirection.json

@@ -11533,6 +11533,13 @@
     {
       "source_path": "hub/apps/windows-app-sdk/applifecycle/app-background-task.md",
       "redirect_url": "/windows/apps/windows-app-sdk/applifecycle/background-tasks",
+      "source_path": "hub/apps/desktop/modernize/desktop-to-uwp-distribute.md",
+      "redirect_url": "/windows/msix/desktop/source-code-overview",
+      "redirect_document_id": false
+    },
+    {
+      "source_path": "hub/apps/desktop/modernize/desktop-to-uwp-extend.md",
+      "redirect_url": "/windows/apps/windows-app-sdk/",
       "redirect_document_id": false
     }
   ]