deps: update dependency com.google.cloud:google-cloud-core to v2.8.15 (…

…#1359)

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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) | `2.8.14` -> `2.8.15` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.15/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.15/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.15/compatibility-slim/2.8.14)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-core/2.8.15/confidence-slim/2.8.14)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

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

### [`v2.8.15`](https://togithub.com/googleapis/java-core/blob/HEAD/CHANGELOG.md#&#8203;2815-httpsgithubcomgoogleapisjava-corecomparev2814v2815-2022-09-23)

[Compare Source](https://togithub.com/googleapis/java-core/compare/v2.8.14...v2.8.15)

##### Dependencies

-   Update dependency com.google.api.grpc:proto-google-iam-v1 to v1.6.0 ([#&#8203;925](https://togithub.com/googleapis/java-core/issues/925)) ([a5475c5](https://togithub.com/googleapis/java-core/commit/a5475c5333ca97392c9e64a883826ea1bd976a01))
-   Update dependency com.google.api.grpc:proto-google-iam-v1 to v1.6.1 ([#&#8203;926](https://togithub.com/googleapis/java-core/issues/926)) ([3bc96fb](https://togithub.com/googleapis/java-core/commit/3bc96fbba4907d4ac3d24915870132fed596c0d3))

</details>

---

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samples/install-without-bom/pom.xml

@@ -36,7 +36,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-core</artifactId>
-      <version>2.8.14</version>
+      <version>2.8.15</version>
     </dependency>
     <dependency>
       <groupId>com.google.cloud</groupId>

samples/snapshot/pom.xml

@@ -35,7 +35,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-core</artifactId>
-      <version>2.8.14</version>
+      <version>2.8.15</version>
     </dependency>
     <dependency>
       <groupId>com.google.cloud</groupId>