GHA: use XCode 15.1 on Mac

* should fix these errors/warnings:

error MT0180: This version of Microsoft.iOS requires the iOS 17.2 SDK (shipped with Xcode 15.1). Either upgrade Xcode to get the required header files or set the managed linker behaviour to Link Framework SDKs Only in your project's iOS Build Options > Linker Behavior (to try to avoid the new APIs). [/Users/runner/work/oxyplot-maui/oxyplot-maui/Source/OxyplotMauiSample/OxyplotMauiSample.csproj::TargetFramework=net8.0-ios]

warning MT0079: The recommended Xcode version for Microsoft.MacCatalyst 17.2.8078 is Xcode 15.1 or later. The current Xcode version (found in /Applications/Xcode_15.0.1.app/Contents/Developer) is 15.0.1. [/Users/runner/work/oxyplot-maui/oxyplot-maui/Source/OxyplotMauiSample/OxyplotMauiSample.csproj::TargetFramework=net8.0-maccatalyst]

Author: janusw

.github/workflows/dotnet.yml

@@ -52,6 +52,10 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: 8.0.x
+    - name: Setup XCode
+      uses: maxim-lobanov/setup-xcode@v1
+      with:
+        xcode-version: '15.1'
     - name: Install .NET MAUI
       run: |
         dotnet nuget locals all --clear