release: update security policy to include 4.2

Update security policy documentation to indicate 4.2 is the latest
version, and drop 4.0 from the supported versions

Signed-off-by: Dan Kalowsky <dkalowsky@amperecomputing.com>
Signed-off-by: Daniel DeGrasse <ddegrasse@tenstorrent.com>

Author: danieldegrasse

.github/SECURITY.md

@@ -8,11 +8,11 @@ updates:
   - The most recent release, and the release prior to that.
   - Active LTS releases.
 
-At this time, with the latest release of v4.0, the supported
+At this time, with the latest release of v4.2, the supported
 versions are:
 
-  - v4.1: Current release
-  - v4.0: Prior release
+  - v4.2: Current release
+  - v4.1: Prior release
   - v3.7: Current LTS
 
 ## Reporting process