uv-client: switch heuristic freshness lifetime to hard-coded value
The comment in the code explains the bulk of this:
```rust
// We previously computed this heuristic freshness lifetime by
// looking at the difference between the last modified header and
// the response's date header. We then asserted that the cached
// response ought to be "fresh" for 10% of that interval.
//
// It turns out that this can result in very long freshness
// lifetimes[1] that lead to uv caching too aggressively.
//
// Since PyPI sets a max-age of 600 seconds and since we're
// principally just interacting with Python package indices here,
// we just assume a freshness lifetime equal to what PyPI has.
//
// Note though that a better solution here is for the index to
// support proper HTTP caching headers (ideally Cache-Control, but
// Expires also works too, as above).
```
We also remove the `heuristic_percent` field on `CacheConfig`. Since
that's actually part of the cache itself, we bump the simple cache
version.
Finally, we add some more `trace!` calls that should hopefully make
diagnosing issues related to the freshness lifetime a bit easier in the
future.
Fixes #5351
