Update supported versions in SECURITY.md

Author: kdinev

SECURITY.md

@@ -4,18 +4,17 @@
 
 | Version  | Supported          |
 | -------- | ------------------ |
-| 19.1.x   | :white_check_mark: |
+| 21.0.x   | :white_check_mark: |
+| 20.1.x   | :white_check_mark: |
+| 20.0.x   | :x:                |
+| 19.2.x   | :white_check_mark: |
+| 19.1.x   | :x:                |
 | 19.0.x   | :x:                |
 | 18.2.x   | :white_check_mark: |
 | 18.1.x   | :x:                |
 | 18.0.x   | :x:                |
 | 17.2.x   | :white_check_mark: |
-| 17.1.x   | :x:                |
-| 17.0.x   | :x:                |
-| 16.1.x   | :white_check_mark: |
-| 16.0.x   | :x:                |
-| 15.1.x   | :white_check_mark: |
-| < 15.1.x | :x:                |
+| <= 17.1.x| :x:                |
 
 ## Reporting a Vulnerability