[next] Update RSC header in has routes (#8651)

### Related Issues

x-ref: vercel/next.js#40979

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packages/next/src/server-build.ts

@@ -1134,7 +1134,7 @@ export async function serverBuild({
   if (appPathRoutesManifest) {
     // create .rsc variant for app lambdas and edge functions
     // to match prerenders so we can route the same when the
-    // __flight__ header is present
+    // __rsc__ header is present
     const edgeFunctions = middleware.edgeFunctions;
 
     for (let route of Object.values(appPathRoutesManifest)) {
@@ -1393,7 +1393,7 @@ export async function serverBuild({
               has: [
                 {
                   type: 'header',
-                  key: '__flight__',
+                  key: '__rsc__',
                 },
               ],
               dest: path.posix.join('/', entryDirectory, '/$1.rsc'),

packages/next/test/fixtures/00-app-dir/vercel.json

@@ -15,7 +15,7 @@
       "path": "/dashboard",
       "status": 200,
       "headers": {
-        "__flight__": "1"
+        "__rsc__": "1"
       },
       "mustContain": "M1:{",
       "mustNotContain": "<html"