deps: update dependency com.google.api:gax-bom to v2.6.1 (#600)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | `2.6.0` -> `2.6.1` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.1/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.1/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.1/compatibility-slim/2.6.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.1/confidence-slim/2.6.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.6.1`](https://togithub.com/googleapis/gax-java/releases/v2.6.1)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v2.6.0...v2.6.1)

##### Bug Fixes

-   declare depenencies of API surfaces as api ([#&#8203;1535](https://www.togithub.com/googleapis/gax-java/issues/1535)) ([#&#8203;1536](https://www.togithub.com/googleapis/gax-java/issues/1536)) ([8794b5c](https://www.github.com/googleapis/gax-java/commit/8794b5c322659af5a339c40552f16d71b7605cdf))

</details>

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Author: renovate-bot

pom.xml

@@ -151,7 +151,7 @@
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-core-parent</site.installationModule>
-    <gax.version>2.6.0</gax.version>
+    <gax.version>2.6.1</gax.version>
     <google.api-common.version>2.0.5</google.api-common.version>
     <google.common-protos.version>2.6.0</google.common-protos.version>
     <google.iam.version>1.1.6</google.iam.version>