deps: use google-cloud-spanner-bom (#258)

Author: suztomo

pom.xml

@@ -70,8 +70,8 @@
     <dependencies>
       <dependency>
         <groupId>com.google.cloud</groupId>
-        <artifactId>libraries-bom</artifactId>
-        <version>12.0.0</version>
+        <artifactId>google-cloud-spanner-bom</artifactId>
+        <version>2.0.2</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>