Target net6.0 only for unit test

[vanillajonathan]

We do not need to target both net472 and net6.0 for the unit tests. Not targeting net472 allows the unit test to also run on Linux without Mono installed.

[codecov]

Codecov Report

Merging #371 (1a28525) into master (07a8058) will not change coverage.
The diff coverage is n/a.

❗ Current head 1a28525 differs from pull request most recent head 01d4ae0. Consider uploading reports for the commit 01d4ae0 to get more accurate results

@@           Coverage Diff           @@
##           master     #371   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files           6        6           
  Lines         839      839           
  Branches       85       85           
=======================================
  Hits          793      793           
  Misses         34       34           
  Partials       12       12           

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