github: security: remove v3.6 from the list of supported releases

Remove v3.6 from the list of supported releases. Zephyr v3.6.0 reached
end-of-life on 2024-11-29.

Fixes: ea8bd5a

Signed-off-by: Henrik Brix Andersen <[email protected]>

Author: henrikbrixandersen

.github/SECURITY.md

@@ -12,8 +12,7 @@ At this time, with the latest release of v4.0, the supported
 versions are:
 
   - v4.0: Current release
-  - v3.7: Current LTS
-  - v3.6: Prior release
+  - v3.7: Prior release and Current LTS
   - v2.7: Prior LTS
 
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