Document supported VFX Reference Platform versions

[doug-walker]

Our CI currently tests four VFX Reference Platform versions: the current year and three previous years. This aligns with what the VFX Platform recommends and should appear somewhere in the documentation. Basically this means that the current version needs to build against the VFX Platform tool chain from three years ago.

Related issues:

• Should we use the VFX Platform as a way to increment various requirements, for example C++ language version? Currently, these increments are handled as separate decisions.
• It is becoming difficult to test even three years back on GitHub Actions.
• We don't test the VFX Platform on Mac and Windows to the extent that it is tested on Linux.
• We may want to specify which OCIO releases correspond to which VFX Platforms.
• Do we want to make any statements about how many years of previous releases "we actively support"? (And how would we define that?) Of course, since the project is open source, people are always free to fix an issue in a previous release on their own. For reference, OIIO only "actively supports" one year back.

For reference, OTIO's statement is here.