Update patch version for release - new version is 1.5.21-dev

Change-Id: I2cbb5b47a3e208835fcc1981e4beb3a86e8736ce
diff --git a/src/main/java/com/android/tools/r8/Version.java b/src/main/java/com/android/tools/r8/Version.java
index 588967b..53ba91f 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 = "1.5.20-dev";
+  public static final String LABEL = "1.5.21-dev";
 
   private Version() {
   }