For years win32-headers.h has "helpfully" set _WIN32_WINNT if it has not
already been set. This has caused problems because the value specified
in win32-headers.h falls behind that set in the config files. This
change removes the fallback and makes it an error to not set this macro
in the build config files.

This only affects MinGW builds where it has caused confusion.

Bug: v8:12099
Change-Id: I013aadc307376fa97afa7b7752910f4fa4a6db82
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3272648Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78168}