Fix style errors in build

[pownkel]

Last month, a regression in the dotnet SDK caused our builds to start breaking due to style errors that should have been silenced. This PR uses a known workaround (setting properties related to the failing style rules to true instead of true:warning. This will not interfere with the workflow described in #340 (i.e. removing the line dotnet_diagnostic.RULE_ID.severity = suggestion will re-enable the rule as expected).

This PR does silence one rule that was not previously silenced, because it refers to a language feature new to dotnet 8, and we build for both dotnet 8 and dotnet 6.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.73%. Comparing base (ee3c4f5) to head (e9e40d1).
Report is 147 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #521   +/-   ##
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  Coverage   59.73%   59.73%           
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  Files         248      248           
  Lines        7572     7572           
  Branches      901      900    -1     
=======================================
  Hits         4523     4523           
  Misses       2637     2637           
  Partials      412      412           

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