Update security policy now that 0.21.X is out.

SECURITY.md

@@ -4,6 +4,7 @@
 
 | Version  | Supported          |
 | -------- | ------------------ |
+| 0.21.x   | :white_check_mark: |
 | 0.20.x   | :white_check_mark: |
 | < 0.20.x | :x:                |