chore(deps): update dependency com.google.cloud:google-cloud-spanner to v6.4.2 (#1176)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-spanner](https://togithub.com/googleapis/java-spanner) | `6.4.1` -> `6.4.2` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.4.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.4.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.4.2/compatibility-slim/6.4.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.4.2/confidence-slim/6.4.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/java-spanner</summary>

### [`v6.4.2`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;642-httpswwwgithubcomgoogleapisjava-spannercomparev641v642-2021-05-14)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.4.1...v6.4.2)

</details>

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Author: renovate-bot

samples/install-without-bom/pom.xml

@@ -32,7 +32,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner</artifactId>
-      <version>6.4.1</version>
+      <version>6.4.2</version>
     </dependency>
     <!-- [END spanner_install_without_bom] -->