Set expiry date for the Security Policy (#91)

Include an expiry date in the security policy primarily as a mechanism
to ensure the support timeline can reasonable be updated if no
end-of-life date has yet been set. 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 2024-12-31. If you are reading
+this after that date, try to find an up-to-date version in the official source
+repository.
+
 ## Supported Versions
 
 Only the latest release of the project is supported with security updates.