[next] Update payload flag (#10147)

x-ref: [slack thread](https://vercel.slack.com/archives/C05E6RJTPJS/p1687898295532579)

.changeset/hip-otters-turn.md

@@ -0,0 +1,5 @@
+---
+"@vercel/next": patch
+---
+
+[next] Update payload flag

packages/next/src/index.ts

@@ -1092,7 +1092,7 @@ export const build: BuildV2 = async ({
           operationType: 'Page', // always Page because we're in legacy mode
           shouldAddHelpers: false,
           shouldAddSourcemapSupport: false,
-          supportsMultiPayloads: !!process.env.NEXT_PRIVATE_MULTI_PAYLOAD,
+          supportsMultiPayloads: true,
           framework: {
             slug: 'nextjs',
             version: nextVersion,

packages/next/src/utils.ts

@@ -821,7 +821,7 @@ export async function createLambdaFromPseudoLayers({
     files,
     shouldAddHelpers: false,
     shouldAddSourcemapSupport: false,
-    supportsMultiPayloads: !!process.env.NEXT_PRIVATE_MULTI_PAYLOAD,
+    supportsMultiPayloads: true,
     framework: {
       slug: 'nextjs',
       version: nextVersion,

packages/next/test/fixtures/22-ssg-v2-multi-payload/vercel.json

@@ -2,11 +2,6 @@
   "version": 2,
   "uploadNowJson": true,
   "builds": [{ "src": "package.json", "use": "@vercel/next" }],
-  "build": {
-    "env": {
-      "NEXT_PRIVATE_MULTI_PAYLOAD": "1"
-    }
-  },
   "probes": [
     {
       "path": "/lambda",