fix: example/animal-service/Gemfile to reduce vulnerabilities (#281)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-SINATRA-3150405

Co-authored-by: snyk-bot <[email protected]>

Author: mefellows

example/animal-service/Gemfile

@@ -10,4 +10,4 @@ gem 'rake'
 gem 'rack', '~> 2.2', '>= 2.2.3.1'
 gem 'sqlite3'
 gem 'sequel'
-gem 'sinatra', '>= 2.2.0'
+gem 'sinatra', '>= 2.2.3'