deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.26.0 (#4732)

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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) | `6.25.7` -> `6.26.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.26.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.26.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.26.0/compatibility-slim/6.25.7)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.26.0/confidence-slim/6.25.7)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v6.26.0`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6260-httpsgithubcomgoogleapisjava-spannercomparev6257v6260-2022-07-13)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.25.7...v6.26.0)

##### Features

-   Adding two new fields for Instance create_time and update_time ([#&#8203;1908](https://togithub.com/googleapis/java-spanner/issues/1908)) ([00b3817](https://togithub.com/googleapis/java-spanner/commit/00b38178e851401e293aa457f7ba5ea593a7b7c5))
-   changes to support data, timestamp and arrays in IT tests ([#&#8203;1840](https://togithub.com/googleapis/java-spanner/issues/1840)) ([c667653](https://togithub.com/googleapis/java-spanner/commit/c667653ec380dccbf205e7b419843da11cf4155a))
-   Error Details Improvement ([c8a2184](https://togithub.com/googleapis/java-spanner/commit/c8a2184c51cc92ec35c759eff68e614fc78fb2e6))
-   Error Details Improvement ([#&#8203;1929](https://togithub.com/googleapis/java-spanner/issues/1929)) ([c8a2184](https://togithub.com/googleapis/java-spanner/commit/c8a2184c51cc92ec35c759eff68e614fc78fb2e6))

##### Bug Fixes

-   enable longpaths support for windows test ([#&#8203;1485](https://togithub.com/googleapis/java-spanner/issues/1485)) ([#&#8203;1946](https://togithub.com/googleapis/java-spanner/issues/1946)) ([fd0b845](https://togithub.com/googleapis/java-spanner/commit/fd0b84523535ba583a1b56acbea98835191daa06))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-trace to v2.3.0 ([#&#8203;1934](https://togithub.com/googleapis/java-spanner/issues/1934)) ([2813eb2](https://togithub.com/googleapis/java-spanner/commit/2813eb21c9f168e8dea149e40dac188933c7e2db))

</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>6.25.7</version>
+        <version>6.26.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>