remove usage of "@vercel/request-context" (#71383)
Removes usage of `@vercel/request-context`, because now we have
`@next/request-context` and can get `waitUntil` from there instead.
Dependent on builder changes from
https://github.com/vercel/vercel/pull/12286, which were [released in
`vercel@39.1.2` /
`@vercel/next@4.4.0`](https://github.com/vercel/vercel/pull/12648)
### Testing
We can verify that, when using an older builder (which doesn't provide
`@next/request-context`), the deploy tests for `after` fail, because
they can't access `waitUntil`:
```
# these will fail, except some edge runtime tests
# because they can use FetchEventResult.waitUntil as a fallback
VERCEL_CLI_VERSION='vercel@39.1.1' \
NEXT_TEST_VERSION='https://vercel-packages.vercel.app/next/commits/b1b97b8bbaa2f9d400de42957869d138a54da304/next' \
pnpm test-deploy test/e2e/app-dir/next-after-app-deploy
```
To see the errors, need to inspect the logs and look for this error
message:
https://github.com/vercel/next.js/blob/d1e554be4c0bac8bf28c84a05065c230cb6f9d80/packages/next/src/server/after/after-context.ts#L145
But using `39.1.2`, they will pass, because `@next/request-context` is
present:
```
# these will pass
VERCEL_CLI_VERSION='vercel@39.1.2' \
NEXT_TEST_VERSION='https://vercel-packages.vercel.app/next/commits/b1b97b8bbaa2f9d400de42957869d138a54da304/next' \
pnpm test-deploy test/e2e/app-dir/next-after-app-deploy
```
