Add warning when using old reactMode

Author: devknoll

packages/next/next-server/server/config.ts

@@ -35,6 +35,19 @@ function assignDefaults(userConfig: { [key: string]: any }) {
     delete userConfig.exportTrailingSlash
   }
 
+  if (typeof userConfig.experimental?.reactMode !== 'undefined') {
+    console.warn(
+      chalk.yellow.bold('Warning: ') +
+        'The experimental "reactMode" option has been replaced with "reactRoot". Please update your next.config.js.'
+    )
+    if (typeof userConfig.experimental?.reactRoot === 'undefined') {
+      userConfig.experimental.reactRoot = ['concurrent', 'blocking'].includes(
+        userConfig.experimental.reactMode
+      )
+    }
+    delete userConfig.experimental.reactMode
+  }
+
   const config = Object.keys(userConfig).reduce<{ [key: string]: any }>(
     (currentConfig, key) => {
       const value = userConfig[key]