Pin @babel/preset-react to v7 in generator e2e template

The generator specs' react e2e template installed @babel/preset-react
with no version constraint. @babel/preset-react@8 was published requiring
@babel/core ^8 as a peer, but Shakapacker's generated app uses
@babel/core ^7. npm therefore resolved the unpinned add to v8 and failed
with ERESOLVE, which aborted the whole `npm add` and left react and
react-dom uninstalled. The subsequent bin/shakapacker build then failed
with "Module not found: Can't resolve 'react'", breaking the Generator
specs job on main and every PR.

Pin to @babel/preset-react@^7 to match the app's @babel/core ^7, matching
the existing pin already used in spec/dummy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: justin808

spec/generator_specs/e2e_template/template.rb

@@ -5,7 +5,12 @@
 package_json = PackageJson.new
 
 # install react
-package_json.manager.add(["react", "react-dom", "@babel/preset-react@^7.17.0"])
+# Align with spec/dummy; React 18 and Babel 7 match this template's app stack.
+package_json.manager.add([
+  "react@^18.3.1",
+  "react-dom@^18.3.1",
+  "@babel/preset-react@^7.18.6"
+])
 
 # update webpack presets for react
 package_json.merge! do |pj|