Drop conditional testing on Mono

[atifaziz]

This PR drops testing on Mono, which was there for historical reasons when it was the go-to option for running & using .NET Framework assemblies on non-Windows platforms. This changed with .NET (Core), which now runs on many non-Windows platforms. Also:

Microsoft maintains a modern fork of Mono runtime in the dotnet/runtime repo and has been progressively moving workloads to that fork. That work is now complete, and we recommend that active Mono users and maintainers of Mono-based app frameworks migrate to .NET which includes work from this fork.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.23%. Comparing base (ede868e) to head (0105792).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1081   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files         112      112           
  Lines        3400     3400           
  Branches      962      962           
=======================================
  Hits         3170     3170           
  Misses        214      214           
  Partials       16       16           

Flag	Coverage Δ
	93.23% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.