Add deprecation warning to Runtime config

Author: devjiwonchoi

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

@@ -1119,13 +1119,15 @@ export interface NextConfig {
    * Add public (in browser) runtime configuration to your app
    *
    * @see [Runtime configuration](https://nextjs.org/docs/pages/api-reference/config/next-config-js/runtime-configuration
+   * @deprecated Runtime config is deprecated and will be removed in Next.js 16.
    */
   publicRuntimeConfig?: { [key: string]: any }
 
   /**
    * Add server runtime configuration to your app
    *
    * @see [Runtime configuration](https://nextjs.org/docs/pages/api-reference/config/next-config-js/runtime-configuration
+   * @deprecated Runtime config is deprecated and will be removed in Next.js 16.
    */
   serverRuntimeConfig?: { [key: string]: any }
 

packages/next/src/server/config.ts

@@ -109,6 +109,19 @@ function checkDeprecations(
   silent: boolean,
   dir: string
 ) {
+  warnOptionHasBeenDeprecated(
+    userConfig,
+    'publicRuntimeConfig',
+    `Runtime config is deprecated and the \`publicRuntimeConfig\` configuration option will be removed in Next.js 16.`,
+    silent
+  )
+  warnOptionHasBeenDeprecated(
+    userConfig,
+    'serverRuntimeConfig',
+    `Runtime config is deprecated and the \`serverRuntimeConfig\` configuration option will be removed in Next.js 16.`,
+    silent
+  )
+
   if (userConfig.experimental?.dynamicIO !== undefined) {
     warnOptionHasBeenDeprecated(
       userConfig,