deps: update dependency com.google.api:gax-bom to v2.8.0 (#659)

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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) | `2.7.1` -> `2.8.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.8.0/compatibility-slim/2.7.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.api:gax-bom/2.8.0/confidence-slim/2.7.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.8.0`](https://togithub.com/googleapis/gax-java/blob/HEAD/CHANGELOG.md#&#8203;280-httpswwwgithubcomgoogleapisgax-javacomparev271v280-2022-01-06)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v2.7.1...v2.8.0)

##### Features

-   update DirectPath xds scheme ([#&#8203;1585](https://www.togithub.com/googleapis/gax-java/issues/1585)) ([7915f85](https://www.github.com/googleapis/gax-java/commit/7915f850313ddfdf35332d976d2567f31c2aa8a7))

##### Dependencies

-   update dependency com.google.api.grpc:proto-google-common-protos to v2.7.1 ([#&#8203;1587](https://www.togithub.com/googleapis/gax-java/issues/1587)) ([eb0e9d1](https://www.github.com/googleapis/gax-java/commit/eb0e9d106a64af5f583d75d7291cdc17652fd5e7))
-   update dependency com.google.http-client:google-http-client-bom to v1.41.0 ([eb0e9d1](https://www.github.com/googleapis/gax-java/commit/eb0e9d106a64af5f583d75d7291cdc17652fd5e7))
-   update dependency com.google.protobuf:protobuf-bom to v3.19.2 ([eb0e9d1](https://www.github.com/googleapis/gax-java/commit/eb0e9d106a64af5f583d75d7291cdc17652fd5e7))
-   upgrade common-protos to 2.7.0 ([#&#8203;1579](https://www.togithub.com/googleapis/gax-java/issues/1579)) ([0a10f5f](https://www.github.com/googleapis/gax-java/commit/0a10f5fe77ab26b3ac7d4c590360945ead72eca1))

##### [2.7.1](https://www.github.com/googleapis/gax-java/compare/v2.7.0...v2.7.1) (2021-12-02)

##### Bug Fixes

-   fix gRPC code conversion ([#&#8203;1555](https://www.togithub.com/googleapis/gax-java/issues/1555)) ([09b99d5](https://www.github.com/googleapis/gax-java/commit/09b99d591497b44c3c25b1a54abb0f1cb69d7376))
-   pass error message when creating ApiException ([#&#8203;1556](https://www.togithub.com/googleapis/gax-java/issues/1556)) ([918ae41](https://www.github.com/googleapis/gax-java/commit/918ae419f84ad5721638ca10eca992333e9f7c3d))
-   revert generics syntax change in MockHttpService test utility ([#&#8203;1574](https://www.togithub.com/googleapis/gax-java/issues/1574)) ([b629488](https://www.github.com/googleapis/gax-java/commit/b629488ffc7d68158158d9197695158f97229c7b))
-   update exception mapping on HTTP error responses ([#&#8203;1570](https://www.togithub.com/googleapis/gax-java/issues/1570)) ([8a170d1](https://www.github.com/googleapis/gax-java/commit/8a170d19b42e9b13d4c69dcfbe531d4d4ca69c90))

##### Dependencies

-   update grpc to 1.42.1 ([#&#8203;1559](https://www.togithub.com/googleapis/gax-java/issues/1559)) ([92b7632](https://www.github.com/googleapis/gax-java/commit/92b76325d54604c98c798c489b3a963fdf21a75c))
-   upgrade protobuf to 3.19.1 ([#&#8203;1571](https://www.togithub.com/googleapis/gax-java/issues/1571)) ([7b354e7](https://www.github.com/googleapis/gax-java/commit/7b354e73b8ce49008bed51076afb255ca5dc68e4))

</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>2.7.1</gax.version>
+    <gax.version>2.8.0</gax.version>
     <google.api-common.version>2.1.1</google.api-common.version>
     <google.common-protos.version>2.7.1</google.common-protos.version>
     <google.iam.version>1.2.0</google.iam.version>