clarify supported versions

SECURITY.md

@@ -11,15 +11,16 @@ submissions, when describing the vulnerability (see https://mapserver.org/develo
 ## Supported Versions
 
 The MapServer PSC (Project Steering Committee) will release patches for security vulnerabilities 
-for the last release branch of the two most recent major releases.  For example, 
-once 8.0 is released, then only 8.0.x and 7.6.x will be patched.
+for the last release branch of the two most recent release series (such as 8.x, 7.x. 6.x, etc.)  For example, 
+once 8.0 is released, then only 8.0.x and 7.6.x will be supported/patched.
 
 Currently, the following versions are supported:
 
 | Version | Supported          |
 | ------- | ------------------ |
 | 7.6.x   | :white_check_mark: |
-| 7.4.x   | :white_check_mark: |
+| 7.4.x   | :x:                |
 | 7.2.x   | :x:                |
 | 7.0.x   | :x:                |
-| < 7.0   | :x:                |
+| 6.4.x   | :white_check_mark: |
+| < 6.4   | :x:                |