[llvm][docs] Update the Python version requirement to 3.8.0 for lit testing on...

[llvm][docs] Update the Python version requirement to 3.8.0 for lit testing on Windows with substitute (virtual) drives. (#81663)

Following the changes made for:
- https://reviews.llvm.org/D154130: [lit][clang] Avoid realpath on Windows due to MAX_PATH limitations
in commit:
- 05d613ea

Python 3.8.0 or newer is now required by at least the following tests
when they are run on Windows from a substitute (virtual) drive. A
substitute drive is often used as a workaround for `MAX_PATH`
limitations on Windows. These tests are impacted because they use the
lit `%{?:real}` path expansion syntax to expand symbolic links and
substitute drives. This path expansion is implemented with Python's
`os.path.realpath()` function which changed behavior in Python 3.8.0
with regard to expansion of substitute drives. The changes mentioned
above rely on the newer Python behavior.
- `clang/test/Lexer/case-insensitive-include-absolute.c`
- `clang/test/Lexer/case-insensitive-include-win.c`

This change updates the LLVM Getting Started guide to note this newer
Python version dependency for this relatively niche case. Python 3.6.0
remains the minimum required Python version otherwise.