Run tests on all platforms; remove .NET 6 setup

Author: paulirwin

.github/workflows/dotnet.yml

@@ -8,15 +8,14 @@ on:
 
 jobs:
   build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v2
-    - name: Setup .NET 6
-      uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: 6.0.x
     - name: Setup .NET 8
       uses: actions/setup-dotnet@v1
       with: