deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.30.2 (#976)

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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) | `6.30.1` -> `6.30.2` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.30.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.30.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.30.2/compatibility-slim/6.30.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.30.2/confidence-slim/6.30.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.30.2`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6302-httpsgithubcomgoogleapisjava-spannercomparev6301v6302-2022-09-21)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.30.1...v6.30.2)

##### Dependencies

-   Update dependency com.google.cloud:google-cloud-monitoring to v3.4.5 ([#&#8203;2022](https://togithub.com/googleapis/java-spanner/issues/2022)) ([0536962](https://togithub.com/googleapis/java-spanner/commit/0536962df9af3feed237f758a560c24fafd81d60))
-   Update dependency org.junit.vintage:junit-vintage-engine to v5.9.1 ([#&#8203;2023](https://togithub.com/googleapis/java-spanner/issues/2023)) ([3fb4235](https://togithub.com/googleapis/java-spanner/commit/3fb423571c1128b7cafdc6596d5366268d74f0e4))

</details>

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Author: renovate-bot

pom.xml

@@ -62,7 +62,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.30.1</version>
+        <version>6.30.2</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>