deps: update gax.version to v2 (major) (#418)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.api:gax-grpc](https://togithub.com/googleapis/gax-java) | `1.67.0` -> `2.0.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-grpc/2.0.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-grpc/2.0.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-grpc/2.0.0/compatibility-slim/1.67.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-grpc/2.0.0/confidence-slim/1.67.0)](https://docs.renovatebot.com/merge-confidence/) |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | `1.67.0` -> `2.0.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.0.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.0.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.0.0/compatibility-slim/1.67.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.0.0/confidence-slim/1.67.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.0.0`](https://togithub.com/googleapis/gax-java/blob/master/CHANGELOG.md#&#8203;200-httpswwwgithubcomgoogleapisgax-javacomparev1670v200-2021-07-30)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v1.67.0...v2.0.0)

##### Features

-   promote to 2.0.0 ([#&#8203;1444](https://www.github.com/googleapis/gax-java/issues/1444)) ([776b1aa](https://www.github.com/googleapis/gax-java/commit/776b1aa73022bedec55e69732245b73cd04608f8))

##### Bug Fixes

-   stop overriding default grpc executor ([#&#8203;1355](https://www.github.com/googleapis/gax-java/issues/1355)) ([b1f8c43](https://www.github.com/googleapis/gax-java/commit/b1f8c43cc90eb8e5ef78d142878841689356738c))

##### Dependencies

-   update api-common, guava, google-auth-library-credentials ([#&#8203;1442](https://www.github.com/googleapis/gax-java/issues/1442)) ([2925ed7](https://www.github.com/googleapis/gax-java/commit/2925ed78cfb74db07a87da28839aeebc9027ac72))

</details>

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pom.xml

@@ -56,7 +56,7 @@
     <site.installationModule>${project.artifactId}</site.installationModule>
 
     <grpc.version>1.39.0</grpc.version>
-    <gax.version>1.67.0</gax.version>
+    <gax.version>2.0.0</gax.version>
     <grpc-gcp.version>1.1.0</grpc-gcp.version>
     <guava.version>30.1.1-jre</guava.version>
     <protobuf.version>3.17.3</protobuf.version>