Set expiry date for the Security Policy (#1328)

Include an expiry date in the security policy primarily as a mechanism
to ensure the supported version range can reasonable be updated if no
end-of-life date has yet been set for a version.

Additionally, it nudges anyone looking at the policy to search for an
up-to-date version - which is more helpful for anyone.

SECURITY.md

@@ -7,6 +7,10 @@ appreciate your efforts to responsibly disclose your findings. Due to the
 non-funded and open-source nature of the project, we take a best-efforts
 approach when it comes to engaging with security reports.
 
+This document should be considered expired after 2024-10-01. If you are
+reading this after that date you should try to find an up-to-date version
+in the official source repository.
+
 ## Supported Versions
 
 The table below shows which versions of the project are currently supported