fix: samples/client/echo_api/java/restclient/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGSPRINGFRAMEWORK-10345766

Author: snyk-bot

samples/client/echo_api/java/restclient/pom.xml

@@ -280,7 +280,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>