update libraries-bom (#6799)

Author: elharo

google-cloud-pubsub/README.md

@@ -19,7 +19,7 @@ If you are using Maven with a BOM, add this to your pom.xml file.
   <dependency>
     <groupId>com.google.cloud</groupId>
     <artifactId>libraries-bom</artifactId>
-    <version>2.8.0</version>
+    <version>2.9.0</version>
     <type>pom</type>
     <scope>import</scope>
    </dependency>