Fix stboot regression and set Go 1.16 as minimum requirement
Created by: sylv-io
u-root requires Go version >= 1.15: ref: https://github.com/u-root/u-root#usage stboot requires Go version >= 1.16: commit: 35b5fcf
Created by: sylv-io
u-root requires Go version >= 1.15: ref: https://github.com/u-root/u-root#usage stboot requires Go version >= 1.16: commit: 35b5fcf