deps: update spring.version to v2.5.6 (#729)

* deps: update spring.version to v2.5.6

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: renovate-bot

samples/spring-framework/pom.xml

@@ -19,7 +19,7 @@ limitations under the License.
   <properties>
     <maven.compiler.target>1.8</maven.compiler.target>
     <maven.compiler.source>1.8</maven.compiler.source>
-    <spring.version>2.5.5</spring.version>
+    <spring.version>2.5.6</spring.version>
   </properties>
 
   <!--