update security policy

the security incorrectly indicated that minor versions would receive backports of security fixes. 
as long as the API is fully backwards-compatible (as indicated by semver), there should be no need to release additional patches for older versions.

Author: piksel

SECURITY.md

@@ -5,10 +5,7 @@
 | Version | Supported          |
 | ------- | ------------------ |
 | 1.3.x   | :white_check_mark: |
-| 1.2.x   | :white_check_mark: |
-| 1.1.x   | :white_check_mark: |
-| 1.0.x   | :white_check_mark: |
-| < 1.0   | :x:                |
+| < 1.3   | :x:                |
 
 ## Reporting a Vulnerability