deps: update bigtable.version to v2.8.0 (#3640)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-bigtable-deps-bom](https://togithub.com/googleapis/java-bigtable) | `2.7.0` -> `2.8.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-deps-bom/2.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-deps-bom/2.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-deps-bom/2.8.0/compatibility-slim/2.7.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-deps-bom/2.8.0/confidence-slim/2.7.0)](https://docs.renovatebot.com/merge-confidence/) |
| [com.google.cloud:google-cloud-bigtable-bom](https://togithub.com/googleapis/java-bigtable) | `2.7.0` -> `2.8.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-bom/2.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-bom/2.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-bom/2.8.0/compatibility-slim/2.7.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-bigtable-bom/2.8.0/confidence-slim/2.7.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.8.0`](https://togithub.com/googleapis/java-bigtable/blob/HEAD/CHANGELOG.md#&#8203;280-httpsgithubcomgoogleapisjava-bigtablecomparev270v280-2022-05-27)

[Compare Source](https://togithub.com/googleapis/java-bigtable/compare/v2.7.0...v2.8.0)

##### Features

-   split emulator into core without deps and a higher level wrapper with grpc helpers ([#&#8203;1264](https://togithub.com/googleapis/java-bigtable/issues/1264)) ([6fdc2c1](https://togithub.com/googleapis/java-bigtable/commit/6fdc2c1a33e140466734debed22d50a7be726d87))

##### Bug Fixes

-   **java:** enable skipped tests for Native Image testing  ([#&#8203;1234](https://togithub.com/googleapis/java-bigtable/issues/1234)) ([560a501](https://togithub.com/googleapis/java-bigtable/commit/560a5018087c6cdc46e2c6150e6ed41075007177))

</details>

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pom.xml

@@ -55,7 +55,7 @@ limitations under the License.
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 
     <!-- core dependency versions -->
-    <bigtable.version>2.7.0</bigtable.version>
+    <bigtable.version>2.8.0</bigtable.version>
     <bigtable-client-core.version>1.26.3</bigtable-client-core.version>
     <grpc-conscrypt.version>2.5.1</grpc-conscrypt.version>
     <!--  keeping at this version to align with hbase-->