deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.25.7 (#4590)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-spanner-bom](https://togithub.com/googleapis/java-spanner) | `6.25.6` -> `6.25.7` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.7/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.7/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.7/compatibility-slim/6.25.6)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.7/confidence-slim/6.25.6)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.25.7`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6257-httpsgithubcomgoogleapisjava-spannercomparev6256v6257-2022-06-30)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.25.6...v6.25.7)

##### Dependencies

-   update dependency com.google.cloud:google-cloud-shared-dependencies to v2.13.0 ([#&#8203;1924](https://togithub.com/googleapis/java-spanner/issues/1924)) ([dde5ee8](https://togithub.com/googleapis/java-spanner/commit/dde5ee8c5fcef36b415929aa32931dc811036eb4))
-   update dependency org.graalvm.buildtools:junit-platform-native to v0.9.12 ([#&#8203;1906](https://togithub.com/googleapis/java-spanner/issues/1906)) ([1800cd9](https://togithub.com/googleapis/java-spanner/commit/1800cd917c26934768296253cbbcf7c91c54afef))

</details>

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Author: renovate-bot

google-cloud-bom/pom.xml

@@ -765,7 +765,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.25.6</version>
+        <version>6.25.7</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>