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4510a236 - Default to latest supported Python version for version-related syntax errors (#17529)

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).