[Slider] Make `index` public in the ValueLabel props (#21932)
[Slider] Make `index` public in the ValueLabel props (#21932)

Co-authored-by: Olivier Tassinari <2824cdc333e4fc36261fd6810f150cfc6830135d@gmail.com>
callemall/material-ui,mbrookes/material-ui,mui-org/material-ui,mbrookes/material-ui,oliviertassinari/material-ui,rscnt/material-ui,rscnt/material-ui,callemall/material-ui,callemall/material-ui,mbrookes/material-ui,callemall/material-ui,oliviertassinari/material-ui,oliviertassinari/material-ui,mui-org/material-ui,mui-org/material-ui,rscnt/material-ui