fix: example/animal-service/Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-RACK-7416898

Author: snyk-bot

example/animal-service/Gemfile

@@ -7,7 +7,7 @@ group :development, :test do
 end
 
 gem 'rake'
-gem 'rack', '>= 3.0.0'
+gem 'rack', '>= 3.1.5'
 gem 'sqlite3'
 gem 'sequel'
 gem 'sinatra', '>= 4.0.0'