deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.8.0 (#517)

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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) | `6.7.0` -> `6.8.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.8.0/compatibility-slim/6.7.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.8.0/confidence-slim/6.7.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.8.0`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;680-httpswwwgithubcomgoogleapisjava-spannercomparev670v680-2021-06-29)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.7.0...v6.8.0)

##### Features

-   add gRPC-GCP channel pool as an option ([#&#8203;1227](https://www.github.com/googleapis/java-spanner/issues/1227)) ([1fa95a9](https://www.github.com/googleapis/java-spanner/commit/1fa95a9993ea8c7a5f943ab39eced4ced4cb87e7))
-   spanner JSON type ([#&#8203;1260](https://www.github.com/googleapis/java-spanner/issues/1260)) ([b2a56c6](https://www.github.com/googleapis/java-spanner/commit/b2a56c68695b6209e20f9f86d83d7c5a0f39c7a8))

##### Bug Fixes

-   Add `shopt -s nullglob` to dependencies script ([#&#8203;1256](https://www.github.com/googleapis/java-spanner/issues/1256)) ([d1712f7](https://www.github.com/googleapis/java-spanner/commit/d1712f7c51752c2359045e5eabac8fc0530a2421))

</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>6.7.0</version>
+        <version>6.8.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>