deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.88.0 (#6971)

Author: renovate-bot

google-cloud-bom/pom.xml

@@ -238,7 +238,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.87.0</version>
+        <version>6.88.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>