GH-45930: [C++] Don't use ICU C++ API in Azure SDK C++ (#45952)

### Rationale for this change

ICU 75.1 or later requires C++17 but Azure SDK for C++ still uses C++14. Azure SDK for C++ includes ICU headers via libxml2. 

### What changes are included in this PR?

We don't need to use ICU C++ API. So we can disable ICU C++ API.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #45930

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -5510,6 +5510,12 @@ function(build_azure_sdk)
   set(BUILD_SAMPLES FALSE)
   set(BUILD_TESTING FALSE)
   set(BUILD_WINDOWS_UWP TRUE)
+  # ICU 75.1 or later requires C++17 but Azure SDK for C++ still uses
+  # C++14. So we disable C++ API in ICU.
+  #
+  # We can remove this after
+  # https://github.com/Azure/azure-sdk-for-cpp/pull/6486 is merged.
+  string(APPEND CMAKE_CXX_FLAGS " -DU_SHOW_CPLUSPLUS_API=0")
   set(CMAKE_UNITY_BUILD FALSE)
   set(DISABLE_AZURE_CORE_OPENTELEMETRY TRUE)
   set(ENV{AZURE_SDK_DISABLE_AUTO_VCPKG} TRUE)