fix: bump .NET version to 8.0.x in build pipeline

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 6.0.x
+        dotnet-version: 8.0.x
     - name: Clean
       run: dotnet clean OpenTok.sln --configuration ${{ env.CONFIGURATION }} && dotnet nuget locals all --clear
     - name: Install dependencies