GitHub: Use macos-15 runner image w/ Xcode 16.4

(macos-13 is deprecated.)

.github/workflows/build.yml

@@ -49,7 +49,7 @@ jobs:
           aws s3 sync --acl public-read --delete $HOME/src/ljt.nightly/latest/files/ s3://libjpeg-turbo-pr/${{env.BRANCH}}/${{github.job}}/
   macos:
     if: ${{!contains(github.ref, 'refs/tags/')}}
-    runs-on: macos-13
+    runs-on: macos-15
     steps:
       - name: Set global environment variables
         run: |
@@ -59,7 +59,7 @@ jobs:
       - name: Set up build
         run: |
           brew install yasm md5sha1sum
-          sudo xcode-select -s /Applications/Xcode_14.2.app
+          sudo xcode-select -s /Applications/Xcode_16.4.app
           mkdir -p $HOME/src/ljt.nightly
           git clone --depth=1 https://github.com/libjpeg-turbo/buildscripts.git -b $BRANCH $HOME/src/buildscripts
       - name: Configure GPG signing