deps: update dependency com.google.api:gax-bom to v2.12.0 (#698)

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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) | `2.11.0` -> `2.12.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.0/compatibility-slim/2.11.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.0/confidence-slim/2.11.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.12.0`](https://togithub.com/googleapis/gax-java/blob/HEAD/CHANGELOG.md#&#8203;2120-httpsgithubcomgoogleapisgax-javacomparev2110v2120-2022-01-28)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v2.11.0...v2.12.0)

##### Features

-   add REST interceptors infrastructure ([#&#8203;1607](https://togithub.com/googleapis/gax-java/issues/1607)) ([0572eed](https://togithub.com/googleapis/gax-java/commit/0572eed8aa29879c74794b22e8ae79e414dd5821))

</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>2.11.0</gax.version>
+    <gax.version>2.12.0</gax.version>
     <google.api-common.version>2.1.3</google.api-common.version>
     <google.common-protos.version>2.7.2</google.common-protos.version>
     <google.iam.version>1.2.1</google.iam.version>