[Breaking] Rename instrumentation onRequestError `context.routeType` from `middleware` to `proxy` (#85006)
Follow up on https://github.com/vercel/next.js/pull/84710,
`instrumentation` docs had `onRequestError` function's param
`context.routeType` has value of `'middleware'`. As it's a user-facing
value, update it to `'proxy'`.