[core] Update SECURITY.md to account for v5
[core] Update SECURITY.md to account for v5

Only support security updates for the latest 2 major versions.
mui-org/material-ui,oliviertassinari/material-ui,callemall/material-ui,callemall/material-ui,rscnt/material-ui,rscnt/material-ui,oliviertassinari/material-ui,mui-org/material-ui,callemall/material-ui,mui-org/material-ui,callemall/material-ui,mbrookes/material-ui,mbrookes/material-ui,rscnt/material-ui,mbrookes/material-ui,oliviertassinari/material-ui