CI: Fix build on macOS 15 runner with CMake 4.0 and Xcode 16.4

- Enforce min CMake version targeted for libssh2
- Downgrade Xcode to 16.2 as that's the last version before upgrading to LLVM 19
  which seems to break building libgit2's bundled zlib

.github/workflows/build.yml

@@ -70,6 +70,10 @@ jobs:
   macos-universal:
     runs-on: macos-15
     steps:
+      - name: Setup Xcode 16.2 (16.3+ incompatible with libgit2's zlib)
+        uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: '16.2'
       - uses: actions/checkout@v4
         with:
           submodules: recursive