uv-client: switch heuristic freshness lifetime to hard-coded value

[BurntSushi]

The comment in the code explains the bulk of this:

// 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. And 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

[charliermarsh]

Thanks @BurntSushi!