chore(java-spanner): modernize submodule pom.xml

Author: chingor13

java-spanner/benchmarks/pom.xml

@@ -60,9 +60,8 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-monitoring</artifactId>
-      <version>${google.cloud.monitoring.version}</version>
+      <version>3.88.0</version><!-- {x-version-update:google-cloud-monitoring:current} -->
     </dependency>
-    <!-- OpenTelemetry test dependencies -->
     <dependency>
       <groupId>io.opentelemetry</groupId>
       <artifactId>opentelemetry-sdk</artifactId>