Use :unprocessable_content in generated Devise config for Rack 3.1+, avoid Rack warnings (#5797)

In Rack v3.1.0, the symbol for HTTP status code 422 was changed from `:unprocessable_entity` to `:unprocessable_content`.

As a result, when using rack 3.2 with the following configuration in `config/initializers/devise.rb`, a warning is shown on login failure:

```ruby
# config/initializers/devise.rb
Devise.setup do |config|
  ...
  config.responder.error_status = :unprocessable_entity
```

Warning message:

```sh
/path-to-app/vendor/bundle/ruby/3.4.0/gems/devise-4.9.4/lib/devise/failure_app.rb:80: warning: Status code :unprocessable_entity is deprecated and will be removed in a future version of Rack. Please use :unprocessable_content instead.
```

This warning can be resolved by updating the config as follows:
```diff
# config/initializers/devise.rb
Devise.setup do |config|
  ...
+  config.responder.error_status = :unprocessable_content
-  config.responder.error_status = :unprocessable_entity
```

This fixes the root cause of the warning for new apps by adjusting the generated config during `$ rails generate devise:install` depending on the rack version, so new apps using newer Rack versions generate `error_status = :unprocessable_content` instead of `:unprocessable_entity`.

Existing apps are handled by [latest versions of Rails, which will now transparently convert the code under the hood to avoid the Rack warning](https://github.com/rails/rails/pull/53383), and Devise will use that translation layer when available in the failure app to prevent the warning there as well (since that isn't covered by Rails automatic conversion).

Signed-off-by: Carlos Antonio da Silva <carlosantoniodasilva@gmail.com>

test/generators/devise_generator_test.rb

@@ -37,5 +37,4 @@ class DeviseGeneratorTest < Rails::Generators::TestCase
     FileUtils.mkdir_p(destination)
     FileUtils.cp routes, destination
   end
-
 end

test/generators/install_generator_test.rb

@@ -23,4 +23,12 @@ class InstallGeneratorTest < Rails::Generators::TestCase
     assert_no_file "config/initializers/devise.rb"
     assert_no_file "config/locales/devise.en.yml"
   end
+
+  test "responder error_status based on rack version" do
+    run_generator(["--orm=active_record"])
+
+    error_status = Rack::RELEASE >= "3.1" ? :unprocessable_content : :unprocessable_entity
+
+    assert_file "config/initializers/devise.rb", /config\.responder\.error_status = #{error_status.inspect}/
+  end
 end