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

…erabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMGOOGLECODEGSON-1730327

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

@@ -277,7 +277,7 @@
         <maven.compiler.target>${java.version}</maven.compiler.target>
         <gson-fire-version>1.8.3</gson-fire-version>
         <swagger-annotations-version>1.5.22</swagger-annotations-version>
-        <retrofit-version>2.5.0</retrofit-version>
+        <retrofit-version>2.10.0</retrofit-version>
             <rxjava-version>3.0.4</rxjava-version>
         <threetenbp-version>1.4.0</threetenbp-version>
         <javax-annotation-version>1.3.2</javax-annotation-version>