chore(deps): update dependency com.google.cloud:libraries-bom to v15 (#581)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:libraries-bom](https://togithub.com/GoogleCloudPlatform/cloud-opensource-java) | major | `14.4.1` -> `15.0.0` |

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samples/snippets/pom.xml

@@ -33,7 +33,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>libraries-bom</artifactId>
-        <version>14.4.1</version>
+        <version>15.0.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>