Add global.json to ignore pre-releases like .Net 5 preview

[sandcastle]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

Need to add global.json to ignore prerelease versions to allow builds to pass.

See discussion here: #113

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #114 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #114   +/-   ##
=======================================
  Coverage   84.01%   84.01%           
=======================================
  Files          47       47           
  Lines        1126     1126           
  Branches      161      161           
=======================================
  Hits          946      946           
  Misses        128      128           
  Partials       52       52           

Flag	Coverage Δ
#unittests	84.01% <ø> (ø)

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