[core] Fix double redirection

Author: oliviertassinari

docs/public/_redirects

@@ -28,7 +28,7 @@
 # To be removed 3+ years after being introduced
 # 2019
 /css-in-js/* /styles/:splat 301
-/customization/css-in-js/ /system/styles/basics/ 301
+/customization/css-in-js/ https://v6.mui.com/system/styles/basics/ 301
 /customization/overrides/ /material-ui/customization/how-to-customize/ 301
 /demos/selection-controls/ /material-ui/react-radio-button/ 301
 /demos/* /components/:splat 301
@@ -40,7 +40,7 @@
 /lab/* /components/:splat 301
 /layout/basics/ /material-ui/guides/responsive-ui/ 301
 /layout/breakpoints/ /material-ui/customization/breakpoints/ 301
-/layout/css-in-js/ /system/styles/basics/ 301
+/layout/css-in-js/ https://v6.mui.com/system/styles/basics/ 301
 /layout/* /components/:splat 301
 /page-layout-examples/* /material-ui/getting-started/templates/:splat 301
 /style/color/ /material-ui/customization/color/ 301
@@ -50,7 +50,7 @@
 /utils/modals/ /material-ui/react-modal/ 301
 /utils/popovers/ /material-ui/react-popover/ 301
 /utils/* /components/:splat 301
-/api/mui-theme-provider/ /system/styles/api/ 301
+/api/mui-theme-provider/ https://v6.mui.com/system/styles/api/ 301
 /components/expansion-panels/ /material-ui/react-accordion/ 301
 /api/expansion-panel/ /material-ui/api/accordion/ 301
 /:lang/api/expansion-panel/ /:lang/material-ui/api/accordion/ 301