Bump versions of dotnet used in CI (#534)

Author: adamreeve

.github/workflows/ci.yml

@@ -132,7 +132,7 @@ jobs:
       if: runner.os == 'macOS'
       run: brew install bison
 
-    # .NET Core Setup (and also MSBuild for Windows).
+    # .NET Setup (and also MSBuild for Windows).
     - name: Setup .NET SDK v8.0.x
       uses: actions/setup-dotnet@v4
       with:
@@ -232,15 +232,10 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-22.04, ubuntu-22.04-arm64, macos-13, macos-14, windows-2022]
-        dotnet: [netcoreapp3.1, net6.0, net7.0, net8.0]
+        dotnet: [net6.0, net8.0, net9.0]
         include:
         - os: windows-2022
           dotnet: net472
-        exclude:
-        - os: ubuntu-22.04-arm64
-          dotnet: netcoreapp3.1
-        - os: macos-14
-          dotnet: netcoreapp3.1
       fail-fast: false
     name: Test NuGet package (${{ matrix.dotnet }} on ${{ matrix.os }})
     runs-on: ${{ matrix.os }}
@@ -253,25 +248,20 @@ jobs:
       with:
         name: nuget-package
         path: nuget
-    - name: Setup .NET Core SDK v3.1.x
-      if: matrix.dotnet == 'netcoreapp3.1'
-      uses: actions/setup-dotnet@v4
-      with:
-        dotnet-version: 3.1.x
     - name: Setup .NET SDK v6.0.x
       if: matrix.dotnet == 'net6.0'
       uses: actions/setup-dotnet@v4
       with:
         dotnet-version: 6.0.x
-    - name: Setup .NET SDK v7.0.x
-      if: matrix.dotnet == 'net7.0'
-      uses: actions/setup-dotnet@v4
-      with:
-        dotnet-version: 7.0.x
     - name: Setup .NET SDK v8.0.x
       uses: actions/setup-dotnet@v4
       with:
         dotnet-version: 8.0.x
+    - name: Setup .NET SDK v9.0.x
+      if: matrix.dotnet == 'net9.0'
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: 9.0.x
     - name: Add local NuGet feed
       run: |
         dotnet new nugetconfig