repo: promote app-next to main example app

This renames packages to make the new frontend system the default:

- packages/app → packages/app-legacy (example-app-legacy)
- packages/app-next → packages/app (example-app)
- packages/app-next-example-plugin → packages/app-example-plugin

Updated all related configuration, scripts, and documentation.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

packages/app/public/index.html

@@ -42,7 +42,7 @@
       href="<%= publicPath %>/safari-pinned-tab.svg"
       color="#5bbad5"
     />
-    <title><%= config.getOptionalString('app.title') ?? 'Backstage' %></title>
+    <title><%= config.getString('app.title') %></title>
 
     <% if (config.has('app.datadogRum')) { %>
     <script>