Use crypto module to generate prefetch token

These tokens don't need to be cryptographically secure (yet) but
eventually they will be so might as well use an actual hash function
to generate them.

packages/next/src/server/app-render/collect-segment-data.tsx

@@ -11,6 +11,7 @@ import { createFromReadableStream } from 'react-server-dom-webpack/client.edge'
 // eslint-disable-next-line import/no-extraneous-dependencies
 import { renderToReadableStream } from 'react-server-dom-webpack/server.edge'
 
+import { createHash } from 'crypto'
 import {
   streamFromBuffer,
   streamToBuffer,
@@ -432,7 +433,8 @@ function createSegmentAccessToken(
   // performing dynamic navigations, to support auth checks in a layout that
   // conditionally renders its slots. At that point we'll need to create an
   // actual cryptographic hash with a salt.
-  return Buffer.from(parentSegmentPathStr + parallelRouteKey, 'utf-8')
-    .toString('hex')
-    .slice(0, 7)
+  return createHash('sha1')
+    .update(parentSegmentPathStr)
+    .update(parallelRouteKey)
+    .digest('hex')
 }