Stabilize swrDelta config (#66108)
Alternative to https://github.com/vercel/next.js/pull/65867 this
stabilizes our `swrDelta` config that allows customizing the
`stale-while-revalidate` period that is included in the `Cache-Control`
header for ISR enabled routes.
This is not a breaking change itself as no default value is provided as
is the case already.
