Change target framework of tests to .NET 6

[asbjornu]

Change target framework of the test project from netcoreapp2.1 to net6.0. Hopefully this will make the tests succeed again.

[codecov]

Codecov Report

Merging #133 (aee55e0) into main (80273e1) will decrease coverage by 0.99%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   68.79%   67.79%   -1.00%     
==========================================
  Files          23       23              
  Lines        4063     3866     -197     
  Branches     1033     1084      +51     
==========================================
- Hits         2795     2621     -174     
+ Misses       1124     1051      -73     
- Partials      144      194      +50     

Impacted Files	Coverage Δ
src/json-ld.net/Core/Regex.cs	59.57% <0.00%> (-40.43%)	⬇️
src/json-ld.net/Core/RDFDatasetUtils.cs	50.53% <0.00%> (-4.31%)	⬇️
src/json-ld.net/Impl/TurtleRDFParser.cs	51.27% <0.00%> (-4.16%)	⬇️
src/json-ld.net/Core/JsonLdProcessor.cs	68.36% <0.00%> (-0.32%)	⬇️
src/json-ld.net/Core/Context.cs	89.27% <0.00%> (-0.29%)	⬇️
src/json-ld.net/Core/NormalizeUtils.cs	95.29% <0.00%> (-0.18%)	⬇️
src/json-ld.net/Util/JavaCompat.cs	70.43% <0.00%> (-0.16%)	⬇️
src/json-ld.net/Core/JsonLdApi.cs	84.84% <0.00%> (-0.13%)	⬇️
src/json-ld.net/Core/DocumentLoader.cs	100.00% <0.00%> (ø)
src/json-ld.net/Impl/TurtleTripleCallback.cs	0.00% <0.00%> (ø)
... and 3 more

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[tpluscode]

Tests pass. Mission accomplished