Use .NET 8 SDK

Author: bording

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Install .NET SDK
         uses: actions/setup-dotnet@v3.0.3
         with:
-          dotnet-version: 7.0.x
+          dotnet-version: 8.0.x
       - name: Build
         run: dotnet build LibGit2Sharp.sln --configuration Release
       - name: Upload packages