Use .NET 8 but still target .NET 4

Author: maddie480

.github/workflows/build-libs.yml

@@ -283,7 +283,7 @@ jobs:
     - uses: actions/setup-dotnet@v4
       if: ${{ matrix.platform == 'linux' }}
       with:
-        dotnet-version: '4'
+        dotnet-version: '8'
 
     - name: Build FNA
       if: ${{ matrix.platform == 'linux' }}