chore: update plugin version in template (googleapis#2959)

The updated version has passed CI in
googleapis/google-cloud-java#10931

Author: JoeWang1127

library_generation/templates/root-pom.xml.j2

@@ -58,7 +58,7 @@
             <!-- The root project runs nexus-staging:release task -->
             <groupId>org.sonatype.plugins</groupId>
             <artifactId>nexus-staging-maven-plugin</artifactId>
-            <version>1.6.13</version>
+            <version>1.7.0</version>
             <extensions>true</extensions>
             <configuration>
               <serverId>sonatype-nexus-staging</serverId>

library_generation/test/resources/test_monorepo_postprocessing/pom-golden.xml

@@ -58,7 +58,7 @@
             <!-- The root project runs nexus-staging:release task -->
             <groupId>org.sonatype.plugins</groupId>
             <artifactId>nexus-staging-maven-plugin</artifactId>
-            <version>1.6.13</version>
+            <version>1.7.0</version>
             <extensions>true</extensions>
             <configuration>
               <serverId>sonatype-nexus-staging</serverId>