[Dep] Small changes to how to pull in Protobuf (grpc#38884)

Two changes are made to accept upcoming Protobuf v30
- Protobuf v30 will introduce a change that removes Bazel+MSVC support unless the `protobuf_allow_msvc option` is explicitly enabled. Therefore, to maintain compatibility, CI needs to define this option.
- CMake file for `protobuf/upb` now depends on the `protobuf_VERSION` variable, which is set in the main protobuf CMake file. A specific `cmake_externalproject` test includes `protobuf/upb` directly and therefore misses the opportunity to define this variable. To address this, the test now defines `protobuf_VERSION` directly.

Closes grpc#38884

COPYBARA_INTEGRATE_REVIEW=grpc#38884 from veblush:protobuf-v30-misc fd01544
PiperOrigin-RevId: 732251635

Author: veblush

examples/cpp/helloworld/cmake_externalproject/CMakeLists.txt

@@ -35,4 +35,12 @@ bazel --version
 
 python3 tools/run_tests/python_utils/bazel_report_helper.py --report_path bazel_rbe
 
-call bazel_rbe/bazel_wrapper.bat --bazelrc=tools/remote_build/windows.bazelrc --output_user_root=T:\_bazel_output test %BAZEL_FLAGS% -- //test/... || exit /b 1
+call bazel_rbe/bazel_wrapper.bat ^
+  --bazelrc=tools/remote_build/windows.bazelrc ^
+  --output_user_root=T:\_bazel_output ^
+  test ^
+  %BAZEL_FLAGS% ^
+  --define=protobuf_allow_msvc=true ^
+  -- ^
+  //test/... ^
+  || exit /b 1