Update dotnet.yml config file:

- Changed property 'dotnet-version': .NET 7 -> .NET 8
- Fixes build issue
- Related to #10

.github/workflows/dotnet.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 7.0.x
+        dotnet-version: 8.0.x
     - name: "Restore: Syllabore"
       run: dotnet restore ./Syllabore/Syllabore/Syllabore.csproj
     - name: "Build: Syllabore"