deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.25.6 (#4516)

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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) | `6.25.5` -> `6.25.6` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.6/compatibility-slim/6.25.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.25.6/confidence-slim/6.25.5)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.25.6`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6256-httpsgithubcomgoogleapisjava-spannercomparev6255v6256-2022-06-22)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.25.5...v6.25.6)

##### Bug Fixes

-   PostgreSQL parser should not treat \ as an escape char ([#&#8203;1921](https://togithub.com/googleapis/java-spanner/issues/1921)) ([260bbe3](https://togithub.com/googleapis/java-spanner/commit/260bbe3cb78e0583975d7085ae5a95dbfd3efd73)), closes [#&#8203;1920](https://togithub.com/googleapis/java-spanner/issues/1920)

##### Documentation

-   **sample:** relocate native image sample from old repo ([#&#8203;1758](https://togithub.com/googleapis/java-spanner/issues/1758)) ([ef187f4](https://togithub.com/googleapis/java-spanner/commit/ef187f4fccaf1c5550e9f6795228e6c7361030db))

##### Dependencies

-   update dependency org.graalvm.buildtools:native-maven-plugin to v0.9.11 ([#&#8203;1907](https://togithub.com/googleapis/java-spanner/issues/1907)) ([01f8a07](https://togithub.com/googleapis/java-spanner/commit/01f8a07c64358368615d8c729c7c47c4b2c687fd))
-   update dependency org.graalvm.buildtools:native-maven-plugin to v0.9.12 ([#&#8203;1918](https://togithub.com/googleapis/java-spanner/issues/1918)) ([be8b50b](https://togithub.com/googleapis/java-spanner/commit/be8b50b56e51245d941c52445498600025e26ba9))

</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>6.25.5</version>
+        <version>6.25.6</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>