deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.18.0 (#734)

Author: renovate-bot

Diff for: pom.xml

@@ -63,7 +63,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.17.4</version>
+        <version>6.18.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>