GHA: update to MacOS 15 and XCode 16

Author: janusw

.github/workflows/dotnet.yml

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