chore(deps): update dependency com.slack.api:slack-app-backend to v1.5.3 (GoogleCloudPlatform#4605)

Author: renovate-bot

functions/slack/pom.xml

@@ -71,7 +71,7 @@
     <dependency>
       <groupId>com.slack.api</groupId>
       <artifactId>slack-app-backend</artifactId>
-      <version>1.5.2</version>
+      <version>1.5.3</version>
     </dependency>
 
     <!-- The following dependencies are only required for testing -->