[docs] Deprecate Python 2 in build instructions

[stevapple]

Deprecate Python 2 in build instructions, since Python 2 is no longer suggested and required (according to #33651).

Resolves SR-NNNN.

[stevapple]

@swift-ci Please smoke test

[stevapple]

CC @compnerd

I think you've meant that Python 2 is no longer needed, including building and using Swift on Windows.

[benrimmington]

#33786 is moving the Ubuntu dependencies (from README.md to docs/HowToGuides/GettingStarted.md).

[stevapple]

@benrimmington But I can do nothing before that PR is merged :(

[benrimmington]

@stevapple You could:

• either submit a review of [docs] Add First Pull Request guide and Getting Started guide. #33786, with your suggested changes for GettingStarted.md,

• or wait until end-of-Monday for [docs] Add First Pull Request guide and Getting Started guide. #33786 to be merged.

[varungandhi-apple]

There are still a couple of unresolved points in #33786 and Monday is a holiday in the US, so it'll probably be end-of-day Tuesday or Wednesday when I land that.

That said, I can update #33786 to reflect the changes here in README.md. (Done) If you changed only WindowsBuild.md in this PR, that would avoid merge conflicts and you could land this whenever it is approved.

[stevapple]

@varungandhi-apple Done.

[compnerd]

Thanks for cleaning this up

[compnerd]

@swift-ci please smoke test