CI: Select Xcode 16.1 explicitly instead of latest-stable

What `latest-stable` means exactly also depends on the image version
we're running the workflow on, and unfortunately this can vary wildly
between GitHub runners.

Fixate the version to 16.1 for now. This version will need to be updated
as soon as we want to increase the minimum supported compiler version.

.github/actions/setup/action.yml

@@ -49,7 +49,7 @@ runs:
       if: ${{ inputs.os == 'macOS' || inputs.os == 'Android' }}
       uses: maxim-lobanov/setup-xcode@v1
       with:
-        xcode-version: latest-stable
+        xcode-version: 16.1
 
     - name: 'Install Dependencies'
       if: ${{ inputs.os == 'macOS' || inputs.os == 'Android' }}