Bump plugin API to 2.12

This fixes the Maven build since the code was already adapted to 2.12.

Change-Id: I5cb2bb2c578bddc2ed0bbda24798437a8f2cf47e
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/pom.xml b/pom.xml
index 9be9335..32d698b 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,7 +26,7 @@
 
   <properties>
     <Gerrit-ApiType>plugin</Gerrit-ApiType>
-    <Gerrit-ApiVersion>2.9</Gerrit-ApiVersion>
+    <Gerrit-ApiVersion>2.12</Gerrit-ApiVersion>
   </properties>
 
   <build>