Don't pre-create asset manifests

This causes a file generation conflict in Rails 7.2, as they seem to
already exist. We don't want to overwrite whatever there is.

```
    conflict    app/assets/config/manifest.js
  Overwrite /Users/pirj/source/rspec-dev/repos/rspec-rails/tmp/example_app/app/assets/config/mani
  Overwrite /Users/pirj/source/rspec-dev/repos/rspec-rails/tmp/example_app/app/assets/config/manifest.js? (enter "h" for help) [Ynaqdhm] y
       force    app/assets/config/manifest.js
    conflict    app/assets/stylesheets/application.css
  Overwrite /Users/pirj/source/rspec-dev/repos/rspec-rails/tmp/example_app/app/assets/stylesheets
  Overwrite /Users/pirj/source/rspec-dev/repos/rspec-rails/tmp/example_app/app/assets/stylesheets/application.css? (enter "h" for help) [Ynaqdhm] y
       force    app/assets/stylesheets/application.css
```

Author: pirj

example_app_generator/generate_app.rb

@@ -67,7 +67,7 @@
 
   copy_file capybara_backport_path, 'spec/support/capybara.rb'
 
-  if Rails::VERSION::STRING > '7'
+  if Rails::VERSION::STRING > '7' && Rails::VERSION::STRING < '7.2'
     create_file 'app/assets/config/manifest.js' do
       "//= link application.css"
     end