Split nav item removal into separate changesets

Use dedicated changesets for the breaking frontend-plugin-api removal,
plugin migrations, and frontend-test-utils update.

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

.changeset/remove-nav-item-blueprint-api.md

@@ -0,0 +1,22 @@
+---
+'@backstage/frontend-plugin-api': minor
+---
+
+**BREAKING**: Removed the deprecated `NavItemBlueprint`. Navigation items are now discovered from `PageBlueprint` extensions based on their `title` and `icon` params.
+
+If you were still using `NavItemBlueprint`, migrate by moving `title` and `icon` to your `PageBlueprint` instead:
+
+```diff
+-const navItem = NavItemBlueprint.make({
+-  params: { title: 'Example', icon: ExampleIcon, routeRef },
+-});
+ const page = PageBlueprint.make({
+   params: {
++    title: 'Example',
++    icon: <ExampleIcon fontSize="inherit" />,
+     routeRef,
+     path: '/example',
+     loader: () => import('./Page').then(m => <m.Page />),
+   },
+ });
+```

.changeset/remove-nav-item-blueprint-plugins.md

@@ -1,5 +1,4 @@
 ---
-'@backstage/frontend-plugin-api': minor
 '@backstage/plugin-app': patch
 '@backstage/plugin-catalog': patch
 '@backstage/plugin-search': patch
@@ -11,7 +10,8 @@
 '@backstage/plugin-devtools': patch
 '@backstage/plugin-catalog-unprocessed-entities': patch
 '@backstage/plugin-app-visualizer': patch
-'@backstage/frontend-test-utils': patch
 ---
 
-Removed the deprecated `NavItemBlueprint`. Navigation items are now discovered from page extensions via their `title` and `icon` params. The app nav extension still accepts legacy `nav-item` extensions for backward compatibility.
+Removed separate nav item extensions. Sidebar entries are now provided via `title` and `icon` on each plugin's page extension.
+
+The app nav extension still accepts legacy `nav-item` extensions for backward compatibility with older plugin versions.

.changeset/remove-nav-item-blueprint-test-utils.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-test-utils': patch
+---
+
+Updated `renderInTestApp` to render legacy nav-item extensions without relying on the removed `NavItemBlueprint` from `@backstage/frontend-plugin-api`.