deps: update dependency com.google.cloud:google-cloud-core to v2.8.21 (#1001)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-core](https://togithub.com/googleapis/java-core) | `2.8.20` -> `2.8.21` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.21/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.21/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.21/compatibility-slim/2.8.20)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.21/confidence-slim/2.8.20)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.8.21`](https://togithub.com/googleapis/java-core/blob/HEAD/CHANGELOG.md#&#8203;2821-httpsgithubcomgoogleapisjava-corecomparev2820v2821-2022-10-10)

[Compare Source](https://togithub.com/googleapis/java-core/compare/v2.8.20...v2.8.21)

##### Dependencies

-   Update dependency com.google.api.grpc:proto-google-iam-v1 to v1.6.2 ([#&#8203;971](https://togithub.com/googleapis/java-core/issues/971)) ([5d778fc](https://togithub.com/googleapis/java-core/commit/5d778fc0d78e67cbcc4eb061da3b66dd3cab440e))
-   Update dependency com.google.api.grpc:proto-google-iam-v1 to v1.6.3 ([#&#8203;974](https://togithub.com/googleapis/java-core/issues/974)) ([1b7fcb9](https://togithub.com/googleapis/java-core/commit/1b7fcb978da55f8d30cf66941bc8208853fef116))

</details>

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Author: renovate-bot

samples/install-without-bom/pom.xml

@@ -36,7 +36,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-core</artifactId>
-      <version>2.8.20</version>
+      <version>2.8.21</version>
     </dependency>
     <dependency>
       <groupId>junit</groupId>

samples/snapshot/pom.xml

@@ -35,7 +35,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-core</artifactId>
-      <version>2.8.20</version>
+      <version>2.8.21</version>
     </dependency>
     <dependency>
       <groupId>junit</groupId>