Switch from Xcode 14.3.1 to 15.4.

Because GitHub [rolled out / is rolling out] an update to the macos-14
image that removes 14.3.1.

I have to admit I'm a little fuzzy on the implications of switching to
15.4 with regard to OS support and the changes of successful linking with binaries
produced by older versions.

.github/workflows/build.yml

@@ -84,7 +84,7 @@ jobs:
       unreal-engine-zip: "s3://cesium-unreal-engine/UE_52_macOS.zip"
       unreal-program-name: "UE_5.2"
       upload-package-base-name: "CesiumForUnreal-52-apple"
-      xcode-version: "14.3.1"
+      xcode-version: "15.4"
   Combine52:
     runs-on: ubuntu-latest
     needs: [Windows52, Linux52, Android52, Apple52]
@@ -202,7 +202,7 @@ jobs:
       unreal-engine-zip: "s3://cesium-unreal-engine/5.3.0/UE_53_macOS.zip"
       unreal-program-name: "UE_5.3"
       upload-package-base-name: "CesiumForUnreal-53-apple"
-      xcode-version: "14.3.1"
+      xcode-version: "15.4"
   Combine53:
     runs-on: ubuntu-latest
     needs: [Windows53, Android53, Linux53, Apple53]
@@ -320,7 +320,7 @@ jobs:
       unreal-engine-zip: "s3://cesium-unreal-engine/5.4.0/UE_54_macOS.zip"
       unreal-program-name: "UE_5.4"
       upload-package-base-name: "CesiumForUnreal-54-apple"
-      xcode-version: "14.3.1"
+      xcode-version: "15.4"
   Combine54:
     runs-on: ubuntu-latest
     needs: [Windows54, Android54, Linux54, Apple54]