Introduce a --fork-strategy preference mode (#9868)

## Summary

This PR makes the behavior in https://github.com/astral-sh/uv/pull/9827
the default: we try to select the latest supported package version for
each supported Python version, but we still optimize for choosing fewer
versions when stratifying by platform.

However, you can opt out with `--fork-strategy fewest`.

Closes https://github.com/astral-sh/uv/issues/7190.

crates/uv-python/src/python_version.rs

@@ -50,7 +50,9 @@ impl schemars::JsonSchema for PythonVersion {
                 ..schemars::schema::StringValidation::default()
             })),
             metadata: Some(Box::new(schemars::schema::Metadata {
-                description: Some("A Python version specifier, e.g. `3.7` or `3.8.0`.".to_string()),
+                description: Some(
+                    "A Python version specifier, e.g. `3.11` or `3.12.4`.".to_string(),
+                ),
                 ..schemars::schema::Metadata::default()
             })),
             ..schemars::schema::SchemaObject::default()