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

…v3.0.4 (#1197)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-spanner-bom](https://togithub.com/googleapis/java-spanner) | patch | `3.0.3` -> `3.0.4` |

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### Release Notes

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

### [`v3.0.4`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;304-httpswwwgithubcomgoogleapisjava-spannercomparev303v304-2020-11-17)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v3.0.3...v3.0.4)

</details>

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pom.xml

@@ -506,7 +506,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>3.0.3</version>
+        <version>3.0.4</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>