deps: update dependency com.google.cloud:gapic-libraries-bom to v1.15.0 (#6086)

Author: renovate-bot

google-cloud-bom/pom.xml

@@ -167,7 +167,7 @@
         <!-- Artifacts from google-cloud-java monorepo -->
         <groupId>com.google.cloud</groupId>
         <artifactId>gapic-libraries-bom</artifactId>
-        <version>1.14.0</version>
+        <version>1.15.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>