Set Gerrit to 3.12.0-rc6

Change-Id: I85d765102b40a53ee493fb41b48a7a4b05b2a5c0
diff --git a/Version.mk b/Version.mk
index 44af295..dc157f4 100644
--- a/Version.mk
+++ b/Version.mk
@@ -1,4 +1,4 @@
 VERSION=3.12.0
-WAR_VERSION=3.12.0-rc5
+WAR_VERSION=3.12.0-rc6
 BRANCH=stable-3.12
-RELEASE=0rc5
+RELEASE=0rc6