Unify allow-runtime with Partial Prefetching (#96106)
Removes the "allow-runtime" prefetch config, and turns its behavior on
implicitly wherever Partial Prefetching is enabled.
The original motivation for "allow-runtime" was to give apps more
control over server costs triggered by prefetches. Until a route
explicitly opts in, prefetches would only be served from the CDN, not
from the server. The problem, though, was it was very confusing to know
when to add or remove this configuration. The incentive for many apps
was to add it everywhere, with no clear signal for when to remove it.
Our updated thinking is that Partial Prefetching itself already provides
sufficient protection against runaway prefetching costs: per-link
prefetches only happen on Link components that explicitly opt in with
the prefetch prop.
The optimizations landed earlier in this stack also make allow-runtime
less necessary: on pages where all the content is statically renderable,
prefetches are served from the static cache and no runtime request is
ever issued; only a page that accesses non-static data is prefetched at
runtime.
The upshot of this decision is that runtime versus static becomes an
internal optimization; the same content gets prefetched regardless of
whether or how Next.js is able to optimize it.
