deps: update dependency com.google.cloud:google-cloud-spanner-bom to …

…v6.4.2 (#2048)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-spanner-bom](https://github.com/googleapis/java-spanner) | `6.4.1` -> `6.4.2` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.4.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.4.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/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-bom/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://github.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;642-httpswwwgithubcomgoogleapisjava-spannercomparev641v642-2021-05-14)

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

</details>

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pom.xml

@@ -543,7 +543,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.4.1</version>
+        <version>6.4.2</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>