fix(use-cache): remove awaiting revalidation (#92636)
### What?
fixes #74882
The `use cache` directive does not revalidate in the background while
serving (not expired) stale data.
For the static pages, the whole page is blocked during revalidation, and
fresh data is fetched served. While I don't agree with this, as it is
just a duplication of the expire behaviour, this is documented as
intended in the comments in the code.
For the dynamic pages, when the data is flagged for revalidation, the
stale data is blocked while fresh data is fetched. Once the fresh data
is fetched, the function returns the stale data. On refreshing, the
fresh data is served. This is the worst of both worlds, as we are still
waiting for fresh data, and then seeing the stale data.
In both cases there is no background revalidation of data, leading to
slow, unresponsive pages.
I have an example app showing this behaviour here:
https://github.com/awo00/nextjs-use-cache-revalidate
### Why?
In the `use-cache-wrapper`, the `generateCacheEntry()` function is
awaited during revalidation which is blocking the stale data being
returned.
### How?
Removing the await allows the stale data to be returned faster.
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Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
