Create SECURITY.md for security policy

[kimmymuezza-droid]

Added a security policy document outlining supported versions and vulnerability reporting.

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Note

Introduces a project security policy.

• Adds SECURITY.md with a supported versions matrix (5.1.x, 4.0.x supported; 5.0.x and <4.0 not supported)
• Documents the vulnerability reporting process and expectations for updates

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