MinimumRequirements.md: add note on minimum GCC version for CGO on Windows

Add a note to the section on CGO spelling out the GCC version requirements
when using CGO on Windows.

Updates golang/go#75077

Change-Id: If1c25b6b9a155e163c8d930ad40e612a18a00a4e
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/701475
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Commit-Queue: Than McIntosh <thanm@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>