GHA: Use macos-15 for Xcode 16

Will it works this time?

Author: helje5

.github/workflows/swift.yml

@@ -36,7 +36,7 @@ jobs:
       - name: Run Tests
         run: swift test
   xcode26:
-    runs-on: macos-latest
+    runs-on: macos-15
     steps:
       - name: Select Xcode 26-beta
         uses: maxim-lobanov/setup-xcode@v1.6.0