deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.9.0 (#521)

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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) | `6.8.0` -> `6.9.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.9.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.9.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.9.0/compatibility-slim/6.8.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.9.0/confidence-slim/6.8.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.9.0`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;690-httpswwwgithubcomgoogleapisjava-spannercomparev680v690-2021-07-05)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.8.0...v6.9.0)

##### Features

-   add support for tagging to Connection API ([#&#8203;623](https://www.github.com/googleapis/java-spanner/issues/623)) ([5722372](https://www.github.com/googleapis/java-spanner/commit/5722372b7869828e372dec06e80e5b0e7280af61))
-   **spanner:** add leader_options to InstanceConfig and default_leader to Database ([#&#8203;1271](https://www.github.com/googleapis/java-spanner/issues/1271)) ([f257671](https://www.github.com/googleapis/java-spanner/commit/f25767144344f0df67662f1b3ef662902384599a))
-   support setting an async executor provider ([#&#8203;1263](https://www.github.com/googleapis/java-spanner/issues/1263)) ([369c8a7](https://www.github.com/googleapis/java-spanner/commit/369c8a771ec48fa1476236f800b0e8eb5982a33c))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-shared-dependencies to v1.4.0 ([#&#8203;1269](https://www.github.com/googleapis/java-spanner/issues/1269)) ([025e162](https://www.github.com/googleapis/java-spanner/commit/025e162813d6321dabe49e32f00934f9ae334e24))

</details>

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Author: renovate-bot

pom.xml

@@ -71,7 +71,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>6.8.0</version>
+        <version>6.9.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>