recognize macOS 11.1 as big sur (#20038)
recognize macOS 11.1 as big sur (#20038)

Big Sur versions go 11.0, 11.0.1, 11.1 (vs. prior versions that
only used the minor component)

Co-authored-by: Todd Gamblin <6c2f3f485ec2bf47caf8dd15510e24e60f05822c@llnl.gov>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack