Revise security policy and add vulnerability reporting (#21029)

Updated the security policy to include vulnerability reporting guidelines and removed supported versions section.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability in any of our SDKs, tools, services, or repositories, please help us keep the community safe by reporting it responsibly.
+
+You can report security vulnerabilities by emailing **[security@avaloniaui.net](mailto:security@avaloniaui.net)**.
+
+Please avoid disclosing the issue publicly until we have had a reasonable amount of time to investigate and release a patch or mitigation. We review all legitimate reports and will work with you to quickly resolve the issue.
+
+Please note that Avalonia does not operate a bug bounty programme.