[maven-release-plugin] prepare branch release-2.1.0

Author: wcn3

pom.xml

@@ -54,7 +54,7 @@
     <connection>scm:git:git@github.com:GoogleCloudPlatform/DataflowJavaSDK.git</connection>
     <developerConnection>scm:git:git@github.com:GoogleCloudPlatform/DataflowJavaSDK.git</developerConnection>
     <url>git@github.com:GoogleCloudPlatform/DataflowJavaSDK.git</url>
-    <tag>HEAD</tag>
+    <tag>release-2.1.0</tag>
   </scm>
 
   <distributionManagement>