fix: samples/client/petstore/java/retrofit2rx3/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMSQUAREUPOKHTTP3-13961111

Author: snyk-bot

samples/client/petstore/java/retrofit2rx3/pom.xml

@@ -272,7 +272,7 @@
         <maven.compiler.target>${java.version}</maven.compiler.target>
         <gson-fire-version>1.9.0</gson-fire-version>
         <swagger-annotations-version>1.6.3</swagger-annotations-version>
-        <retrofit-version>2.11.0</retrofit-version>
+        <retrofit-version>3.0.0</retrofit-version>
         <rxjava-version>3.0.4</rxjava-version>
         <jakarta-annotation-version>1.3.5</jakarta-annotation-version>
         <beanvalidation-version>2.0.2</beanvalidation-version>