Maintenance: Update security info on supported versions for the upcoming release

SECURITY.md

@@ -4,8 +4,8 @@
 
 | Version  | Supported          |
 | -------- | ------------------ |
-| 6.3.x    | :white_check_mark: |
-| <= 6.2.x | :x:                |
+| 6.4.x    | :white_check_mark: |
+| <= 6.3.x | :x:                |
 
 ## Reporting a Vulnerability