Update SECURITY.md - only supporting 2.1.x now

Author: metaprime

SECURITY.md

@@ -7,7 +7,8 @@ currently being supported with security updates.
 
 | Version | Supported          |
 | ------- | ------------------ |
-| 1.7.x   | :white_check_mark: |
+| 2.1.x   | :white_check_mark: |
+| 1.7.x   | :x:                |
 | < 1.7   | :x:                |
 
 ## Reporting a Vulnerability