Switch to .slnx format

With .NET SDK 9.0.200, the legacy GUID goop in solution files is no longer necessary.

Replace it with a concise, human-readable (and editable) XML format.

Update `publish.yml` to install latest .NET 9 SDK, for `.slnx` support

.github/workflows/publish.yml

@@ -56,7 +56,7 @@ jobs:
                              # https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on
 
     env:
-      Solution_Name: RoyalApps.Community.Rdp.sln                  # Replace with your solution name, i.e. MyWpfApp.sln.
+      Solution_Name: RoyalApps.Community.Rdp.slnx                 # Replace with your solution name, i.e. MyWpfApp.sln.
       # Test_Project_Path: your-test-project-path                 # Replace with the path to your test project, i.e. MyWpfApp.Tests\MyWpfApp.Tests.csproj.
       # Wap_Project_Directory: your-wap-project-directory-name    # Replace with the Wap project directory relative to the solution, i.e. MyWpfApp.Package.
       # Wap_Project_Path: your-wap-project-path                   # Replace with the path to your Wap project, i.e. MyWpf.App.Package\MyWpfApp.Package.wapproj.
@@ -74,7 +74,7 @@ jobs:
     - name: Install .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '9.0'
+        dotnet-version: '9.0.x'
 
     # Add  MSBuild to the PATH: https://github.com/microsoft/setup-msbuild
     - name: Setup MSBuild.exe

src/RoyalApps.Community.Rdp.slnx

@@ -0,0 +1,7 @@
+<Solution>
+  <Folder Name="/Solution Items/">
+    <File Path="..\README.md" />
+  </Folder>
+  <Project Path="RoyalApps.Community.Rdp.WinForms.Demo\RoyalApps.Community.Rdp.WinForms.Demo.csproj" />
+  <Project Path="RoyalApps.Community.Rdp.WinForms\RoyalApps.Community.Rdp.WinForms.csproj" />
+</Solution>