Reduce required Windows CMake version to 3.14 (#89390)

When we landed the CMake minimum requirement constraint for Visual
Studio 2019, we landed it with minimum version 3.15, since that's what
was shipping with the current version of VS 2019 at the time. Looking at
the release notes of earlier versions, it's clear that earlier versions
of Visual Studio 2019 shipped with version 3.14. See:
https://devblogs.microsoft.com/cppblog/visual-studio-cmake-support-clang-llvm-cmake-3-14-vcpkg-and-performance-improvements/

Looking at release notes for CMake 3.15, there are no features/fixes
introduced in that version that we are dependent on.
https://cmake.org/cmake/help/latest/release/3.15.html

Issue: https://github.com/flutter/flutter/issues/88589