Write RFC for cutting backwards-incompatible src CLI release

Created by: eseliger

From the discussion about whether we are cutting a src cli release at some point which reduces backward compatibility:

Once we are working on backend workspace resolution, with the intention being that we’d ship backend workspace resolution and then start a 3(?) month timer after which src-cli would no longer support previous backend versions (which would be heavily advertised). That way, we can move logic in into the backend and don't have to keep the duplicative code around. Also, at that point all current feature flags become invalid, so we can get rid of a lot of additional code paths.