Fix MacOS CI build (#2522)

As of https://github.com/actions/runner-images/issues/12934, the Mac runners use CMake 4.0, which drops compatibility with CMake < 3.5, but the gRPC library version we use is marked as having CMake 3.1 as its minimum version.

Work around this by using `-DCMAKE_POLICY_VERSION_MINIMUM=3.5`.

.github/workflows/autobuild.yaml

@@ -171,6 +171,7 @@ jobs:
             "-DLLVM_ENABLE_ZLIB=FORCE_ON"
             "-DCMAKE_OSX_ARCHITECTURES=x86_64;arm64"
             "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.13"
+            "-DCMAKE_POLICY_VERSION_MINIMUM=3.5"
           # BoringSSL doesn't support universal binaries when building with ASM.
           grpc_cmake: >-
             "-DOPENSSL_NO_ASM=ON"