set dotnet version

Author: wing328

.github/workflows/samples-dotnet-appveyor.yml

@@ -47,7 +47,7 @@ jobs:
       - uses: actions/checkout@v5
       - uses: actions/[email protected]
         with:
-          dotnet-version: '7.0.x'
+          dotnet-version: '3.0.0'
       - name: Build
         working-directory: ${{ matrix.sample }}
         run: dotnet build Org.OpenAPITools.sln