Bump google-cloud-spanner from 6.29.0 to 6.31.0 in /modules/gcloud

Bumps [google-cloud-spanner](https://github.com/googleapis/java-spanner) from 6.29.0 to 6.31.0.
- [Release notes](https://github.com/googleapis/java-spanner/releases)
- [Changelog](https://github.com/googleapis/java-spanner/blob/main/CHANGELOG.md)
- [Commits](googleapis/java-spanner@v6.29.0...v6.31.0)

---
updated-dependencies:
- dependency-name: com.google.cloud:google-cloud-spanner
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

modules/gcloud/build.gradle

@@ -6,7 +6,7 @@ dependencies {
     testImplementation 'com.google.cloud:google-cloud-datastore:2.11.4'
     testImplementation 'com.google.cloud:google-cloud-firestore:3.4.2'
     testImplementation 'com.google.cloud:google-cloud-pubsub:1.120.18'
-    testImplementation 'com.google.cloud:google-cloud-spanner:6.29.0'
+    testImplementation 'com.google.cloud:google-cloud-spanner:6.31.0'
     testImplementation 'com.google.cloud:google-cloud-bigtable:2.11.1'
     testImplementation 'org.assertj:assertj-core:3.23.1'
 }