feat: add a react 17 deprecation warning

Signed-off-by: Paul Schultz <pschultz@pobox.com>

.changeset/fluffy-lands-stand.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': minor
+---
+
+Add a warning for React 17 deprecation that triggers when frontend packages and plugins start.

packages/cli/src/modules/start/commands/package/start/startFrontend.ts

@@ -22,6 +22,7 @@ import {
 } from '../../../../build/lib/bundler';
 import { paths } from '../../../../../lib/paths';
 import { BackstagePackageJson } from '@backstage/cli-node';
+import { hasReactDomClient } from '../../../../build/lib/bundler/hasReactDomClient';
 
 interface StartAppOptions {
   verifyVersions?: boolean;
@@ -40,6 +41,12 @@ export async function startFrontend(options: StartAppOptions) {
     resolvePath(options.targetDir ?? paths.targetDir, 'package.json'),
   )) as BackstagePackageJson;
 
+  if (!hasReactDomClient()) {
+    console.warn(
+      'React 17 is now deprecated! Please follow the Backstage migration guide to update to React 18: https://backstage.io/docs/tutorials/react18-migration/',
+    );
+  }
+
   const waitForExit = await serveBundle({
     entry: options.entry,
     targetDir: options.targetDir,