deps: upgrade gson to 2.8.9 (#1549)

CC: @Neenu1995 @thiagotnunes 

Testing whether "deps" makes a release pull request.

Author: suztomo

google-cloud-spanner/pom.xml

@@ -240,6 +240,7 @@
     <dependency>
       <groupId>com.google.code.gson</groupId>
       <artifactId>gson</artifactId>
+      <!-- explicitly declaring to override the version -->
       <version>2.8.9</version>
     </dependency>
     <dependency>