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

crates/uv-cache/src/lib.rs

@@ -709,7 +709,9 @@ impl CacheBucket {
             Self::FlatIndex => "flat-index-v0",
             Self::Git => "git-v0",
             Self::Interpreter => "interpreter-v2",
-            Self::Simple => "simple-v10",
+            // Note that when bumping this, you'll also need to bump it
+            // in crates/uv/tests/cache_clean.rs.
+            Self::Simple => "simple-v11",
             Self::Wheels => "wheels-v1",
             Self::Archive => "archive-v0",
             Self::Builds => "builds-v0",