fix: samples/client/echo_api/java/feign-gson/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGJETBRAINSKOTLIN-2393744

Author: snyk-bot

samples/client/echo_api/java/feign-gson/pom.xml

@@ -300,7 +300,7 @@
         <java.version>1.8</java.version>
         <maven.compiler.source>${java.version}</maven.compiler.source>
         <maven.compiler.target>${java.version}</maven.compiler.target>
-        <feign-version>13.2.1</feign-version>
+        <feign-version>13.7</feign-version>
         <feign-form-version>3.8.0</feign-form-version>
         <gson-version>2.10.1</gson-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>