Specify dotnet-version in CI workflow

[mandreko-bitwarden]

Description

I found when making other changes to this repository that builds were failing due to the dotnet-version being installed as too new for this project. To complicate matters, the tests run only in dotnet-6 for some reason. This should likely be updated in future efforts.

However, by specifying dotnet8 for build/package and dotnet6 for running tests, the CI does work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.88%. Comparing base (7debaa5) to head (61d8fd2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #188   +/-   ##
=======================================
  Coverage   59.88%   59.88%           
=======================================
  Files          43       43           
  Lines         688      688           
  Branches       56       56           
=======================================
  Hits          412      412           
  Misses        272      272           
  Partials        4        4           

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