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>

package.json

@@ -35,7 +35,6 @@
     "clean": "backstage-cli repo clean",
     "create-plugin": "echo \"use 'yarn new' instead\"",
     "dev": "echo \"use 'yarn start' instead\"",
-    "dev:next": "echo \"use 'yarn start:next' instead\"",
     "docker-build": "yarn tsc && yarn workspace example-backend build && yarn workspace example-backend build-image",
     "fix": "backstage-cli repo fix --publish",
     "postinstall": "husky || true",
@@ -57,8 +56,8 @@
     "start": "backstage-cli repo start",
     "start-backend": "echo \"Use 'yarn start example-backend' instead\"",
     "start:docker": "docker compose -f docker-compose.deps.yml up --wait && BACKSTAGE_ENV=docker yarn start",
+    "start:legacy": "yarn start example-app-legacy example-backend",
     "start:microsite": "cd microsite/ && yarn start",
-    "start:next": "yarn start example-app-next example-backend",
     "storybook": "storybook dev -p 6006",
     "sync-issue-templates": "node ./.github/ISSUE_TEMPLATE/sync.js",
     "techdocs-cli": "node scripts/techdocs-cli.js",