Rename PythonRequest::Any -> PythonRequest::Default

[zanieb]

As we support more complex Python discovery behaviors such as:

• Allow creating venv with free-threaded python builds #7431
• Support pre-releases in Python version requests - command --python <major.minor.pre-release> #7335
• Avoid selecting prerelease Python installations without opt-in #7300

Any is no longer accurate, we actually are looking for a reasonable default Python version to use which may exclude the first one we find. Separately, we need the idea of Any to improve behavior when listing versions (e.g., #7286) where we do actually want to match any Python version. As a first step, we'll rename Any to Default. Then, we'll introduce a new Any that actually behaves as we'd expect.