docs: add maintained releases to security text (runatlantis#4945)

Author: nitrocode

SECURITY.md

@@ -3,3 +3,7 @@
 ## Reporting a Vulnerability
 
 We take security issues seriously. Please report a security vulnerability to the maintainers using [private vulnerability reporting](https://github.com/runatlantis/atlantis/security/advisories/new).
+
+## Maintained releases
+
+Only the two latest minor releases are maintained. For example, if `0.29.7` is the latest, then `0.29.x` and `0.28.x` will receive security fixes.