Update Python to 3.11+ on Windows

[stevapple]

Motivation

Up until Swift 6.0 the toolchain is still relying on Python 3.9, which is released 4 years ago and long outdated. Especially with the introduction of a native ARM64 toolchain, it becomes more of a problem that:

• Python 3.9 was initially picked up because Visual Studio 2022 includes it as a workload. This is no longer true since Visual Studio on ARM64 didn't provide such workload, and on x64 it was already marked as EOL.
• Python.org didn't officially add Windows on ARM support until Python 3.11, which means 3.10 and earlier minors are not officially compatible. This would make it hard for users to install a matching Python if we don't use 3.11 or newer.
• The pythonarm64 NuGet package, though still providing a 3.9 toolset, marked it as deprecated and suggested to upgrade to 3.11 or newer. To keep up with upstream changes, especially security fixes, an upgrade should be ideal.

Proposed solution

Upgrade Python to 3.11 in build.ps1. If possible, switch to use Python.org's released installers as they're easier (and mandatory) to retrieve for regular users.

Alternatives considered

No response

Additional information

No response