Add changelog for 1.2.21

ChangeLog.md

@@ -3,6 +3,13 @@
 <!-- Find: ([^\`/\[])(KT-\d+) -->
 <!-- Replace: $1[`$2`](https://youtrack.jetbrains.com/issue/$2) -->
 
+## 1.2.21
+
+### Fixes
+
+- [`KT-22349`](https://youtrack.jetbrains.com/issue/KT-22349) Android: creating new Basic activity fails with Throwable: "Inconsistent FILE tree in SingleRootFileViewProvider" at SingleRootFileViewProvider.checkLengthConsistency()
+- [`KT-22459`](https://youtrack.jetbrains.com/issue/KT-22459) Remove .proto files from kotlin-reflect.jar
+
 ## 1.2.20
 
 ### Android