fix: samples/openapi3/client/petstore/spring-cloud-3-with-optional/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGSPRINGFRAMEWORKSECURITY-9486467

Author: snyk-bot

samples/openapi3/client/petstore/spring-cloud-3-with-optional/pom.xml

@@ -56,7 +56,7 @@
         <dependency>
             <groupId>org.springframework.security</groupId>
             <artifactId>spring-security-oauth2-client</artifactId>
-            <version>6.1.1</version>
+            <version>6.3.8</version>
         </dependency>
         <dependency>
             <groupId>com.fasterxml.jackson.datatype</groupId>