Add same-origin to fetchServerResponse (#45648)

Mirrors the way the router fetches in `pages`. This is only needed for older browsers.


## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/src/client/components/router-reducer/fetch-server-response.ts

@@ -33,6 +33,8 @@ export async function fetchServerResponse(
   }
 
   const res = await fetch(url.toString(), {
+    // Backwards compat for older browsers. `same-origin` is the default in modern browsers.
+    credentials: 'same-origin',
     headers,
   })
   const canonicalUrl = res.redirected