Add a Supported Versions table to SECURITY.md

Add a section to the security policy that describes the support status of each release, like the [OIIO table](https://github.com/AcademySoftwareFoundation/OpenImageIO/security#supported-versions).

Open SECURITY.md and review the supported-versions table linked in the issue from OpenImageIO as the format reference. Add a section describing the support status of each MaterialX release; done means SECURITY.md contains a complete, accurate table for the project's releases.

Written by the indexing model from the issue text.