Workaround for installation error of BigDecimal 3.1.5 in JRuby

This PR is a workaround for installation error of BigDecimal 3.1.5 in JRuby:

```console
Installing bigdecimal 3.1.5 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
```

https://github.com/rubocop/rubocop-rails/actions/runs/7192052922/job/19587776909

Gemfile

@@ -16,5 +16,19 @@ gem 'simplecov'
 gem 'test-queue'
 gem 'yard', '~> 0.9'
 
+group :test do
+  # FIXME: This is a workaround for installation error of BigDecimal 3.1.5 in JRuby:
+  #
+  # ```
+  # Installing bigdecimal 3.1.5 with native extensions
+  # Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
+  # ```
+  #
+  # https://github.com/rubocop/rubocop-rails/actions/runs/7192052922/job/19587776909
+  #
+  # See: https://github.com/ruby/bigdecimal/issues/279
+  gem 'bigdecimal', '< 3.1.5', platform: :jruby
+end
+
 local_gemfile = File.expand_path('Gemfile.local', __dir__)
 eval_gemfile local_gemfile if File.exist?(local_gemfile)