Suppress a RuboCop offense after running rails new with --devcontainer

This PR suppresses the following RuboCop offense when running `rubocop` after generating a new Rails project with `--devcontainer`:

```console
$ rails new example --devcontainer
$ cd example
$ bin/rubocop
(snip)

Offenses:

test/application_system_test_case.rb:14:1: C: [Correctable] Layout/EmptyLinesAroundClassBody: Extra empty line detected at class body end.
```

This PR removes the following redundant blank line by default that cause the offense:

```diff
 require "test_helper"

 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   if ENV["CAPYBARA_SERVER_PORT"]
     served_by host: "rails-app", port: ENV["CAPYBARA_SERVER_PORT"]

     driven_by :selenium, using: :headless_chrome, screen_size: [ 1400, 1400 ], options: {
       browser: :remote,
       url: "http://#{ENV["SELENIUM_HOST"]}:4444"
     }
   else
     driven_by :selenium, using: :headless_chrome, screen_size: [ 1400, 1400 ]
   end
-
 end
```

This is a similar issue to rails#52199

Author: koic

railties/lib/rails/generators/rails/devcontainer/devcontainer_generator.rb

@@ -150,7 +150,7 @@ def local_rails_mount
         end
 
         def system_test_configuration
-          optimize_indentation(<<-'RUBY', 2)
+          optimize_indentation(<<-'RUBY', 2).chomp
             if ENV["CAPYBARA_SERVER_PORT"]
               served_by host: "rails-app", port: ENV["CAPYBARA_SERVER_PORT"]