deps: update dependency com.google.api:gax-bom to v2.15.0

Author: renovate-bot

pom.xml

@@ -151,7 +151,7 @@
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-core-parent</site.installationModule>
-    <gax.version>2.13.0</gax.version>
+    <gax.version>2.15.0</gax.version>
     <google.api-common.version>2.1.5</google.api-common.version>
     <google.common-protos.version>2.8.0</google.common-protos.version>
     <google.iam.version>1.2.10</google.iam.version>