deps: update dependency com.google.api:gax-bom to v2.6.0 (#594)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.api:gax-bom](https://github.com/googleapis/gax-java) | `2.5.0` -> `2.6.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.0/compatibility-slim/2.5.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.6.0/confidence-slim/2.5.0)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

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

### [`v2.6.0`](https://github.com/googleapis/gax-java/blob/master/CHANGELOG.md#&#8203;260-httpswwwgithubcomgoogleapisgax-javacomparev253v260-2021-10-15)

[Compare Source](https://github.com/googleapis/gax-java/compare/v2.5.0...v2.6.0)

##### Features

-   remove deprecated Generated annotation ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))

##### Bug Fixes

-   Fix com.google.rpc.Code to StatusCode.Code conversion logic ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))

##### Dependencies

-   update api-common to 2.0.5 ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))
-   update auto-value to 1.8.2 ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))
-   update com_google_protobuf to 3.18.1 ([#&#8203;1519](https://www.github.com/googleapis/gax-java/issues/1519)) ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))
-   update google-http-client to 1.40.1 ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))
-   update grpc to 1.41.0 ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))
-   update guava to v31 ([2d76bff](https://www.github.com/googleapis/gax-java/commit/2d76bff6d64da818a3aff7ea0bdf5a36b82c3464))

##### [2.5.3](https://www.github.com/googleapis/gax-java/compare/v2.5.2...v2.5.3) (2021-10-13)

##### Bug Fixes

-   Fix `com.google.rpc.Code` to `StatusCode.Code` conversion  logic ([#&#8203;1508](https://www.github.com/googleapis/gax-java/issues/1508)) ([61b1617](https://www.github.com/googleapis/gax-java/commit/61b161799faf292be1394111381f8a35e757b85a))

##### [2.5.2](https://www.github.com/googleapis/gax-java/compare/v2.5.1...v2.5.2) (2021-10-13)

##### Dependencies

-   release multiple artifacts at once ([#&#8203;1506](https://www.github.com/googleapis/gax-java/issues/1506)) ([8c022f6](https://www.github.com/googleapis/gax-java/commit/8c022f69f7878280e00f200f65a931ff0f8cfe45))

##### [2.5.1](https://www.github.com/googleapis/gax-java/compare/v2.5.0...v2.5.1) (2021-10-08)

##### Dependencies

-   fix release pipeline ([#&#8203;1500](https://www.github.com/googleapis/gax-java/issues/1500)) ([f8ae03b](https://www.github.com/googleapis/gax-java/commit/f8ae03bbf0389d5fd943d214c1058ee012be757b))

</details>

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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>2.5.0</gax.version>
+    <gax.version>2.6.0</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>