[C++20] [Modules] Enable reduced BMI by default
As documented in 20.x, we'd like to keep reduced BMI off by default for
1~2 versions. And now we're in 22.x.
I rarely receive bug reports for reduced BMI. I am not sure about the
reason. Maybe not a lot of people are using it. Or it is really stable
enough.
And also, we've been enabling the reduced BMI internally for roughly half a
year.
So I think it's the time to move on. See the document changes for other
information.
