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

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-RACK-6274383
- https://snyk.io/vuln/SNYK-RUBY-RACK-6274384
- https://snyk.io/vuln/SNYK-RUBY-RACK-6274385

Author: snyk-bot

example/animal-service/Gemfile

@@ -7,7 +7,7 @@ group :development, :test do
 end
 
 gem 'rake'
-gem 'rack', '~> 2.2', '>= 2.2.6.4'
+gem 'rack', '~> 2.2', '>= 2.2.8.1'
 gem 'sqlite3'
 gem 'sequel'
 gem 'sinatra', '>= 4.0.0'