Update patch version for release - new version is 1.6.21-dev

Change-Id: I9798041392276bd1a177b9996bb3153bd51332fb
diff --git a/src/main/java/com/android/tools/r8/Version.java b/src/main/java/com/android/tools/r8/Version.java
index 56682aa..418b038 100644
--- a/src/main/java/com/android/tools/r8/Version.java
+++ b/src/main/java/com/android/tools/r8/Version.java
@@ -11,7 +11,7 @@
 
   // This field is accessed from release scripts using simple pattern matching.
   // Therefore, changing this field could break our release scripts.
-  public static final String LABEL = "1.6.20-dev";
+  public static final String LABEL = "1.6.21-dev";
 
   private Version() {
   }