Add .NET Standard 2.1 framework

[natemcmaster]

Required to enable C# nullable reference type checking.

[codecov]

Codecov Report

Merging #424 (c2bbaec) into main (e67be50) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #424   +/-   ##
=======================================
  Coverage   67.84%   67.84%           
=======================================
  Files         105      105           
  Lines        3281     3281           
=======================================
  Hits         2226     2226           
  Misses       1055     1055           

Impacted Files	Coverage Δ
src/CommandLineUtils/IO/Pager.cs	0.00% <ø> (ø)
src/CommandLineUtils/Internal/Util.cs	33.33% <ø> (ø)
src/CommandLineUtils/Utilities/DotNetExe.cs	56.25% <ø> (ø)

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