GHA: fix Mac build

* by updating to MacOS 14 and XCode 15.4
* there were errors like this:
  warning MT0079: The recommended Xcode version for Microsoft.iOS 17.5.8018 is Xcode 15.4 or later. The current Xcode version (found in /Applications/Xcode_15.1.app/Contents/Developer) is 15.1. [/Users/runner/work/oxyplot-maui/oxyplot-maui/Source/OxyplotMauiSample/OxyplotMauiSample.csproj::TargetFramework=net8.0-ios]
  warning MT0079: The recommended Xcode version for Microsoft.MacCatalyst 17.5.8018 is Xcode 15.4 or later. The current Xcode version (found in /Applications/Xcode_15.1.app/Contents/Developer) is 15.1. [/Users/runner/work/oxyplot-maui/oxyplot-maui/Source/OxyplotMauiSample/OxyplotMauiSample.csproj::TargetFramework=net8.0-maccatalyst]
  error MT0180: This version of Microsoft.iOS requires the iOS 17.5 SDK (shipped with Xcode 15.4). 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]

Author: janusw

.github/workflows/dotnet.yml

@@ -43,7 +43,7 @@ jobs:
         path: ./Source/*/nuget/*.nupkg
 
   macBuild:
-    runs-on: macos-13
+    runs-on: macos-14
     steps:
     - uses: actions/checkout@v4
       with:
@@ -55,7 +55,7 @@ jobs:
     - name: Setup XCode
       uses: maxim-lobanov/setup-xcode@v1
       with:
-        xcode-version: '15.1'
+        xcode-version: '15'
     - name: Install .NET MAUI
       run: |
         dotnet nuget locals all --clear