[use-cache] use computed size of the ReadableStream for LRU evicting (#75607)
### What?
The `default` cache handler does not use the computed size of the
ReadableStream for LRU evicting. Because the default size is `1`, this
ends up allowing up to 50M items in the LRU cache.
### How?
- We now pass `(entry) => entry.size` as a second parameter to the
`LRUCache` constructor
- We also adjust the LRUCache size to 50MB instead of 50M bytes to be
more consistent with other places (e.g. the static paths cache).
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Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>