This is a minimal change to start to require the new minimum bootstrap.
Taking advantage of the newer bootstap to simplify and improve code is
left to be done in separate CLs.

For #64751.

Change-Id: I1d4f883fae3026354a199174d51e79a36cd2dc53
Reviewed-on: https://go-review.googlesource.com/c/go/+/606156


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