Update testproject to net8 and c# to 12

[stefannikolei]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c6fda62) 19% compared to head (9e1388e) 19%.

Additional details and impacted files

@@        Coverage Diff         @@
##           main   #42   +/-   ##
==================================
  Coverage    19%   19%           
==================================
  Files         4     4           
  Lines       346   346           
  Branches     85    85           
==================================
  Hits         68    68           
  Misses      278   278           

Flag	Coverage Δ
unittests	19% <ø> (ø)

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[JimBobSquarePants]

We should update all dependencies listed in the props too.

https://github.com/SixLabors/SharedInfrastructure/tree/c6fda622baf2074516a380fdf88ad6f4bde81537/msbuild