VOL-2828 Bump post-2.3 release

Change-Id: I8965e5e27ce7a24e7a93edad3581e10aaeeb60f4
diff --git a/VERSION b/VERSION
index 3b31fa0..eef2640 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.21
+2.4.0-dev0