Test on xcode 16 only

Author: aashishpatil-g

.github/workflows/spm.yml

@@ -56,7 +56,7 @@ jobs:
         os: [macos-14]
         # GitHub actions' runners do not include visionOS. https://github.com/actions/runner-images/issues/10559
         target: [iOS, macOS, catalyst]
-        xcode: [Xcode_15.4, Xcode_16.2]
+        xcode: [Xcode_16.2]
     runs-on: ${{ matrix.os }}
     env:
       FIREBASECI_USE_LATEST_GOOGLEAPPMEASUREMENT: 1