deps: update dependency com.google.api:gax-bom to v2.12.2 (#719)

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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) | `2.12.0` -> `2.12.2` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.2/compatibility-slim/2.12.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.12.2/confidence-slim/2.12.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.12.2`](https://togithub.com/googleapis/gax-java/blob/HEAD/CHANGELOG.md#&#8203;2122-httpsgithubcomgoogleapisgax-javacomparev2121v2122-2022-02-09)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v2.12.1...v2.12.2)

### [`v2.12.1`](https://togithub.com/googleapis/gax-java/blob/HEAD/CHANGELOG.md#&#8203;2121-httpsgithubcomgoogleapisgax-javacomparev2120v2121-2022-02-09)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v2.12.0...v2.12.1)

</details>

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java-core/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>2.12.0</gax.version>
+    <gax.version>2.12.2</gax.version>
     <google.api-common.version>2.1.4</google.api-common.version>
     <google.common-protos.version>2.7.3</google.common-protos.version>
     <google.iam.version>1.2.3</google.iam.version>