[libc++] Switch to the current XCode beta on macOS builders (#106363)

This unblocks a ton of work including #76756 as it updates to a newer
version of AppleClang.

Author: philnik777

.github/workflows/libcxx-build-and-test.yaml

@@ -208,7 +208,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: 'latest-stable'
+          xcode-version: 'latest'
       - uses: seanmiddleditch/gha-setup-ninja@master
       - name: Build and test
         run: |