GH-45991: [C++] Bump bundled nlohmann_json to v3.12.0 (#46112)

### Rationale for this change

The version we are currently using is pretty old, from 2021.

### What changes are included in this PR?

Bump the version to latest v3.12.0:
https://github.com/nlohmann/json/tree/v3.12.0

Also remove the requirement to use `-DCMAKE_POLICY_VERSION_MINIMUM` when building it.

### Are these changes tested?

Yes via CI 

### Are there any user-facing changes?

No

* GitHub Issue: #45991

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -4324,6 +4324,10 @@ macro(build_nlohmann_json)
       # google-cloud-cpp requires JSON_MultipleHeaders=ON
       -DJSON_BuildTests=OFF -DJSON_MultipleHeaders=ON)
 
+  # We can remove this once we remove -DCMAKE_POLICY_VERSION_MINIMUM=3.5
+  # from EP_COMMON_CMAKE_ARGS.
+  list(REMOVE_ITEM NLOHMANN_JSON_CMAKE_ARGS -DCMAKE_POLICY_VERSION_MINIMUM=3.5)
+
   set(NLOHMANN_JSON_BUILD_BYPRODUCTS ${NLOHMANN_JSON_PREFIX}/include/nlohmann/json.hpp)
 
   externalproject_add(nlohmann_json_ep

cpp/thirdparty/versions.txt

@@ -84,8 +84,8 @@ ARROW_LZ4_BUILD_VERSION=v1.10.0
 ARROW_LZ4_BUILD_SHA256_CHECKSUM=537512904744b35e232912055ccf8ec66d768639ff3abe5788d90d792ec5f48b
 ARROW_MIMALLOC_BUILD_VERSION=v2.0.6
 ARROW_MIMALLOC_BUILD_SHA256_CHECKSUM=9f05c94cc2b017ed13698834ac2a3567b6339a8bde27640df5a1581d49d05ce5
-ARROW_NLOHMANN_JSON_BUILD_VERSION=v3.10.5
-ARROW_NLOHMANN_JSON_BUILD_SHA256_CHECKSUM=5daca6ca216495edf89d167f808d1d03c4a4d929cef7da5e10f135ae1540c7e4
+ARROW_NLOHMANN_JSON_BUILD_VERSION=v3.12.0
+ARROW_NLOHMANN_JSON_BUILD_SHA256_CHECKSUM=4b92eb0c06d10683f7447ce9406cb97cd4b453be18d7279320f7b2f025c10187
 ARROW_OPENTELEMETRY_BUILD_VERSION=v1.13.0
 ARROW_OPENTELEMETRY_BUILD_SHA256_CHECKSUM=7735cc56507149686e6019e06f588317099d4522480be5f38a2a09ec69af1706
 ARROW_OPENTELEMETRY_PROTO_BUILD_VERSION=v0.17.0