Default to latest supported Python version for version-related syntax errors (#17529)

## Summary

This PR partially addresses #16418 via the following:

- `LinterSettings::unresolved_python_version` is now a `TargetVersion`,
which is a thin wrapper around an `Option<PythonVersion>`
- `Checker::target_version` now calls `TargetVersion::linter_version`
internally, which in turn uses `unwrap_or_default` to preserve the
current default behavior
- Calls to the parser now call `TargetVersion::parser_version`, which
calls `unwrap_or_else(PythonVersion::latest)`
- The `Checker`'s implementation of
`SemanticSyntaxContext::python_version` also uses
`TargetVersion::parser_version` to use `PythonVersion::latest` for
semantic errors

In short, all lint rule behavior should be unchanged, but we default to
the latest Python version for the new syntax errors, which should
minimize confusing version-related syntax errors for users without a
version configured.

## Test Plan

Existing tests, which showed no changes (except for printing default
settings).

crates/ruff_server/src/lint.rs

@@ -101,7 +101,8 @@ pub(crate) fn check(
         settings.linter.unresolved_target_version
     };
 
-    let parse_options = ParseOptions::from(source_type).with_target_version(target_version);
+    let parse_options =
+        ParseOptions::from(source_type).with_target_version(target_version.parser_version());
 
     // Parse once.
     let parsed = ruff_python_parser::parse_unchecked(source_kind.source_code(), parse_options)