[Infra] Revert "Start Xcode 16.3 in CI" (#14669)

Author: ncooke3

.github/workflows/abtesting.yml

@@ -23,20 +23,23 @@ jobs:
     strategy:
       matrix:
         include:
+          - os: macos-13
+            xcode: Xcode_15.2
+            target: ios
           - os: macos-14
-            xcode: Xcode_16.2
+            xcode: Xcode_15.4
             target: ios
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: ios
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: tvos
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: macos
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: watchos
     runs-on: ${{ matrix.os }}
     steps:
@@ -84,26 +87,29 @@ jobs:
     strategy:
       matrix:
         include:
+          - os: macos-13
+            xcode: Xcode_15.2
+            target: iOS
           - os: macos-14
-            xcode: Xcode_16.2
+            xcode: Xcode_15.4
             target: iOS
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: iOS
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: tvOS
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: macOS
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: watchOS
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: catalyst
           - os: macos-15
-            xcode: Xcode_16.3
+            xcode: Xcode_16.2
             target: visionOS
     runs-on: ${{ matrix.os }}
     steps:
@@ -207,7 +213,7 @@ jobs:
     # Don't run on private repo.
     if: github.event_name == 'schedule' && github.repository == 'Firebase/firebase-ios-sdk'
 
-    runs-on: macos-15
+    runs-on: macos-14
     strategy:
       matrix:
         target: [ios, tvos, macos]