deps: update iam.version to v1.2.6 (#598)

Author: renovate-bot

first-party-dependencies/pom.xml

@@ -67,7 +67,7 @@
     <google.http-client.version>1.41.2</google.http-client.version>
     <google.oauth-client.version>1.33.1</google.oauth-client.version>
     <google.api-client.version>1.33.1</google.api-client.version>
-    <iam.version>1.2.1</iam.version>
+    <iam.version>1.2.6</iam.version>
   </properties>
 
   <dependencyManagement>