chore: update .NET version in GitHub Actions workflows to 9.0.x

Author: fengb3

.github/workflows/build-and-test.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '9.0.x'
 
     - name: Restore dependencies
       run: dotnet restore

.github/workflows/release.yml

@@ -14,7 +14,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '9.0.x'
 
     - name: Restore dependencies
       run: dotnet restore