deps: update dependency com.google.cloud:google-cloud-shared-dependencies to v0.15.0 (#614)

Author: renovate-bot

pom.xml

@@ -63,7 +63,7 @@
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-spanner-parent</site.installationModule>
-    <google.cloud.shared-dependencies.version>0.14.1</google.cloud.shared-dependencies.version>
+    <google.cloud.shared-dependencies.version>0.15.0</google.cloud.shared-dependencies.version>
   </properties>
 
   <dependencyManagement>