GH-48801: [C++] Set CMAKE_POLICY_VERSION_MINIMUM for RapidJSON (#49993)

### Rationale for this change

We already set this variable when adding projects through `ExternalProject_Add()`, but the config file that RapidJSON exports sets `cmake_minimum_required()` to a version older than 3.5. Set the policy minimum when searching for the just-built RapidJSON.

### What changes are included in this PR?

Set `CMAKE_POLICY_VERSION_MINIMUM` in `FindRapidJSONAlt.cmake`

### Are these changes tested?

Tested locally

### Are there any user-facing changes?

* GitHub Issue: #48801

Authored-by: Kyle Edwards <kyedwards@nvidia.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: KyleFromNVIDIA

cpp/cmake_modules/FindRapidJSONAlt.cmake

@@ -26,7 +26,10 @@ endif()
 if(RapidJSONAlt_FIND_QUIETLY)
   list(APPEND find_package_args QUIET)
 endif()
+set(_CMAKE_POLICY_VERSION_MINIMUM_OLD ${CMAKE_POLICY_VERSION_MINIMUM})
+set(CMAKE_POLICY_VERSION_MINIMUM 3.5)
 find_package(RapidJSON ${find_package_args})
+set(CMAKE_POLICY_VERSION_MINIMUM ${_CMAKE_POLICY_VERSION_MINIMUM_OLD})
 if(RapidJSON_FOUND)
   set(RapidJSONAlt_FOUND TRUE)
   if(NOT TARGET RapidJSON)