Fix CI workflow: install both .NET 8 and 9 SDKs

- Install both .NET 8.0 and 9.0 SDKs for multi-targeting
- Simplify build matrix to only vary by OS
- Fix coverage upload condition

Author: rafalmaciag

.github/workflows/ci.yml

@@ -18,15 +18,16 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        dotnet-version: ['8.0.x', '9.0.x']
 
     steps:
     - uses: actions/checkout@v4
 
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: ${{ matrix.dotnet-version }}
+        dotnet-version: |
+          8.0.x
+          9.0.x
     
     - name: Restore dependencies
       run: dotnet restore
@@ -38,7 +39,7 @@ jobs:
       run: dotnet test --no-build --configuration Release --verbosity normal --logger "console;verbosity=detailed" --collect:"XPlat Code Coverage"
 
     - name: Upload coverage to Codecov
-      if: matrix.os == 'ubuntu-latest' && matrix.dotnet-version == '9.0.x'
+      if: matrix.os == 'ubuntu-latest'
       uses: codecov/codecov-action@v4
       with:
         file: '**/coverage.cobertura.xml'